Introductory Chapter: 4D Imaging

*Jinfeng Yang and Hidehiro Yasuda*

## **1. Introduction**

The study of ultrafast phenomena, including structural dynamics and molecular reactions, is of great interest for physics, chemistry, biology, and materials science. There are numerous examples of phase transitions in condensed materials and chemical reactions in free molecules proceeding on nanosecond, picosecond, and even femtosecond time scales. To study processes or reactions on such intricate scales, more sophisticated apparatus would be needed. It is well known that electron microscopy is a powerful imaging technique and is applied to a wide research field. The progress of electron microscopy has shown that three-dimensional (3D) material structures can be observed with an atomic spatial resolution. However, the conventional electron microscopy does not allow studying ultrafast processes because of the limitation of the speed of video camera.

The study of ultrafast structural dynamics or molecular reactions requires the use of probes ensuring not only high spatial but also high temporal resolutions. For this purpose, the new development of ultrafast electron microscopy (UEM), by combining temporal resolution into conventional electron microscopy, has been begun in the world. UEM uses a short pulsed electron beam replacing the continuous electron beam in the conventional electron microscopy to image the atomic motion by time-resolved recording in real time. By introducing temporal resolution into 3D electron microscopy, UEM allows us to observe the four fundamental dimension structures of matter: three spatial and one temporal, which is called 4D imaging.

Recent developments in UEM have shown that spatial and temporal information of matter can be obtained simultaneously on very small and fast scales. The first UEM was proposed to observe fast processes using a modified 120-keV electron microscope by Ahmed H. Zewail, Nobel Prize winner in Chemistry 1999, in the California Institute of Technology [1, 2]. He and his colleagues succeeded to observe the laser-photon-induced picosecond structural phase transition in vanadium dioxide film using a stroboscopic method with "single" electron pulses [3]. Later, a hybrid 200-keV apparatus was developed. A spatial-temporal resolution of 3.4 Å and 250 fs has been achieved. Recently, there are many research activities focused on improving the electron source and electron optics inside the microscope to achieve better temporal and spatial resolutions [4–9]. However, in the current UEM, the samples must be pumped 107 times or more by the laser. The process being studied must be perfectly reversible. To study the irreversible processes, it is necessary to record images with a larger number of electrons per pulse possible.

In this chapter, we introduce a novel UEM method with relativistic-energy electron pulses. In this relativistic UEM, an advanced radio-frequency (rf) acceleration technology is used to generate relativistic femtosecond electron pulses containing a large number of electrons in pulse and to achieve single-shot femtosecond imaging for the study of ultrafast irreversible structural processes.

## **2. UEM with relativistic femtosecond electron pulses**

The relativistic UEM [10–14] is constructed with three principal components: a rf acceleration-based electron gun, a condenser system, and an imaging system. **Figure 1** shows a photo of the relativistic UEM, which is 3.5 m in height and 0.8 m in diameter. The rf electron gun is driven by a high power of rf to generate a highpeak rf electric field of 100 MV/m, which is 10 times higher than that of direct current gun in the conventional electron microscopy. The electrons emitted from photocathode are then quickly accelerated by the rf electric field into the relativistic energy region to reduce the effect of space charge, yielding ultrashort pulses containing a large number of electrons in pulse. The details of the rf electron gun and the generation of femsecond electron pulses are described in Chapter 2 [15].

Next, the electrons pass through a series of condenser lenses, which use magnetic field to precisely control the intensity of the beam, and its illumination angle on the sample. A relativistic-energy electron imaging system, including an objective lens, an intermediate lens and two projector lenses, is used to magnify the microscopic images. Finally, the images are recorded with a viewing screen (scintillator)

**3**

the followings:

*Introductory Chapter: 4D Imaging*

energy electrons.

*DOI: http://dx.doi.org/10.5772/intechopen.92350*

from the experimental results.

to prepare suitable thin samples.

1.A 100-fs-long pulsed beam containing 106

obtained as 0.027 Å<sup>−</sup><sup>1</sup>

been achieved [20].

has been generated using the rf gun [10–12].

the 3 MeV fs electron pulse is achievable [11, 12].

[19, 20].

advantages over nonrelativistic-energy UEMs:

via a charge-coupled device camera [16]. The relativistic UEM is also an ultra-high voltage transimission electron microscopy (TEM). It exhibits many significant

1.High temporal resolution of 100 fs or less is achievable, because the ultrashort electron pulses of <100 fs can be produced by the rf gun. The transit-time broadening due to the relative energy spread is reduced using the relativistic-

2.The relativistic UEM enables to observe the irreversible processes in materials by single-shot imaging with high-intensity femtosecond electron pulses.

3.The high-energy electrons significantly increase the extinction distance of elastic scattering. Our previous studies [17, 18] indicate that the kinematic theory with the assumption of single elastic scattering events can be applied in the relativistic UEM. This enables one to easily explain structural dynamics

4.A thick sample can be used for measurement, thus obviating the requirement

5.The relativistic UEM is suitable for in situ observations. A large pole piece of

The structural dynamics is observed in UEM with a pump-and-probe method, as shown in **Figure 2**. The femtosecond laser pulse is used as a pump pulse to excite the sample, while the electron pulse is used to record the time evolution of image of the structure by changing the time interval between the electron pulse and the laser pump pulse. The time resolution of UEM is determined mainly by the pulse duration s of the probe electrons and the pump laser. A high temporal resolution can be achieved with the ultrashort electron pulse and the ultrashort laser pump pulse. In this UEM, many demonstrations have been carried out and summarize as

2.In the imaging experiments using these femtosecond pulses, we successfully observed contrast TEM images of 200-nm-diameter gold nanoparticles and other materials. At a low-magnification observation, single-shot imaging with

3.In the electron diffraction measurement, we successfully detected high-

contrast electron diffraction images of single crystalline, polycrystalline, and amorphous materials. An excellent spatial resolution of diffraction images was

4.In the pump-and-probe experiments using the relativistic femtosecond pulses, a laser-induced ultrafast melting dynamics in crystalline gold [17, 18] and a laser-excited ultrafast electronically driven phase transition in single-crystalline silicon [19, 20] were observed. The best temporal resolution of 100 fs has

–107

electrons at an energy of 3 MeV

the objective lens can be applied for installing various specimens.

## *Introductory Chapter: 4D Imaging DOI: http://dx.doi.org/10.5772/intechopen.92350*

*Novel Imaging and Spectroscopy*

**2. UEM with relativistic femtosecond electron pulses**

The relativistic UEM [10–14] is constructed with three principal components: a rf acceleration-based electron gun, a condenser system, and an imaging system. **Figure 1** shows a photo of the relativistic UEM, which is 3.5 m in height and 0.8 m in diameter. The rf electron gun is driven by a high power of rf to generate a highpeak rf electric field of 100 MV/m, which is 10 times higher than that of direct current gun in the conventional electron microscopy. The electrons emitted from photocathode are then quickly accelerated by the rf electric field into the relativistic energy region to reduce the effect of space charge, yielding ultrashort pulses containing a large number of electrons in pulse. The details of the rf electron gun and the generation of femsecond electron pulses are described in Chapter 2 [15]. Next, the electrons pass through a series of condenser lenses, which use magnetic field to precisely control the intensity of the beam, and its illumination angle on the sample. A relativistic-energy electron imaging system, including an objective lens, an intermediate lens and two projector lenses, is used to magnify the microscopic images. Finally, the images are recorded with a viewing screen (scintillator)

*Photo of UEM with relativistic-energy femtosecond electron pulses constructed at Osaka University [13, 14].*

**2**

**Figure 1.**

via a charge-coupled device camera [16]. The relativistic UEM is also an ultra-high voltage transimission electron microscopy (TEM). It exhibits many significant advantages over nonrelativistic-energy UEMs:


The structural dynamics is observed in UEM with a pump-and-probe method, as shown in **Figure 2**. The femtosecond laser pulse is used as a pump pulse to excite the sample, while the electron pulse is used to record the time evolution of image of the structure by changing the time interval between the electron pulse and the laser pump pulse. The time resolution of UEM is determined mainly by the pulse duration s of the probe electrons and the pump laser. A high temporal resolution can be achieved with the ultrashort electron pulse and the ultrashort laser pump pulse. In this UEM, many demonstrations have been carried out and summarize as the followings:


#### **Figure 2.**

*(a) General schematic of UEM using relativistic femtosecond electron pulse and (b) pump-and-probe method for the observation of structural dynamics [13].*

The details of the above experiments have been reported in the related references. The results demonstrate the advantages of relativistic UEM, including access to high-order Bragg reflections, single-shot imaging with the relativistic femtosecond electron pulse, and the feasibility of time-resolved imaging to study ultrafast structural dynamics.

## **3. Conclusion**

Ultrafast electron microscopy with relativistic femtosecond electron pulses is a very promising 4D imaging technique for scientists wishing to study ultrafast structural dynamics in materials. It is an unprecedented innovative technology that enables femtosecond atomic-scale imaging using single-shot measurement and paves the way for the study of irreversible processes in physics, chemistry, biology, and materials science.

The relativistic UEM is also a very compact, ultra-high voltage electron microscopy. It can be used in a variety of settings such as general research institutions and laboratories. Furthermore, by providing a femtosecond temporal resolution, the relativistic UEM will constitute the next generation of electron microscopes. It will allow the study of structural dynamics to be broken into unprecedented timeframes, further encouraging the discovery of new knowledge.

## **Acknowledgements**

The authors acknowledge Prof. Yoshida Y., Drs. Kan K. and Gohdo M. of the Institute of Scientific and Industrial Research in Osaka University for their valuable discussions, Profs Tanimura K. of the Research Center for Ultra-High Voltage Electron Microscopy (UHVEM) in Osaka University for their valuable suggestions. Additionally, the authors thank Urakawa J., Takatomi T., and

**5**

**Author details**

\* and Hidehiro Yasuda<sup>2</sup>

provided the original work is properly cited.

\*Address all correspondence to: yang@sanken.osaka-u.ac.jp

1 The Institute of Scientific and Industrial Research, Osaka University, Osaka, Japan

2 Research Center for Ultra-High Voltage Electron Microscopy, Osaka University,

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

Jinfeng Yang1

Japan

*Introductory Chapter: 4D Imaging*

the fabrication of the rf gun.

*DOI: http://dx.doi.org/10.5772/intechopen.92350*

JP16K13687 of Challenging Research Exploratory, Japan.

Terunuma N. of the High Energy Accelerator Research Organization (KEK) for

This research was funded by JSPS KAKENHI Grant Numbers JP22246127, JP26246026, and JP17H01060 of Grant-in-Aid for Scientific Research (A) and

## *Introductory Chapter: 4D Imaging DOI: http://dx.doi.org/10.5772/intechopen.92350*

*Novel Imaging and Spectroscopy*

structural dynamics.

*for the observation of structural dynamics [13].*

and materials science.

**Acknowledgements**

**3. Conclusion**

**Figure 2.**

The details of the above experiments have been reported in the related references. The results demonstrate the advantages of relativistic UEM, including access to high-order Bragg reflections, single-shot imaging with the relativistic femtosecond electron pulse, and the feasibility of time-resolved imaging to study ultrafast

*(a) General schematic of UEM using relativistic femtosecond electron pulse and (b) pump-and-probe method* 

Ultrafast electron microscopy with relativistic femtosecond electron pulses is a very promising 4D imaging technique for scientists wishing to study ultrafast structural dynamics in materials. It is an unprecedented innovative technology that enables femtosecond atomic-scale imaging using single-shot measurement and paves the way for the study of irreversible processes in physics, chemistry, biology,

The relativistic UEM is also a very compact, ultra-high voltage electron microscopy. It can be used in a variety of settings such as general research institutions and laboratories. Furthermore, by providing a femtosecond temporal resolution, the relativistic UEM will constitute the next generation of electron microscopes. It will allow the study of structural dynamics to be broken into unprecedented time-

The authors acknowledge Prof. Yoshida Y., Drs. Kan K. and Gohdo M. of the Institute of Scientific and Industrial Research in Osaka University for their valuable discussions, Profs Tanimura K. of the Research Center for Ultra-High Voltage Electron Microscopy (UHVEM) in Osaka University for their valuable suggestions. Additionally, the authors thank Urakawa J., Takatomi T., and

frames, further encouraging the discovery of new knowledge.

**4**

Terunuma N. of the High Energy Accelerator Research Organization (KEK) for the fabrication of the rf gun.

This research was funded by JSPS KAKENHI Grant Numbers JP22246127, JP26246026, and JP17H01060 of Grant-in-Aid for Scientific Research (A) and JP16K13687 of Challenging Research Exploratory, Japan.

## **Author details**

Jinfeng Yang1 \* and Hidehiro Yasuda<sup>2</sup>

1 The Institute of Scientific and Industrial Research, Osaka University, Osaka, Japan

2 Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, Japan

\*Address all correspondence to: yang@sanken.osaka-u.ac.jp

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

## **References**

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[2] Zewail AH. Four-Dimensional Electron Microscopy. Science. 2010;**328**:187-193. DOI: 10.1126/ science.1166135

[3] Grinolds MS, Lobastov VA, Weissenrieder J, Zewail AH. Proceedings of the National Academy of Sciences of the United States of America. 2006;**103**:18427-18431. DOI: 10.1073/ pnas.0609233103

[4] Piazza L, Masiel DJ, LaGrange T, Reed BW, Barwick B, Carbone F. Design and implementation of a fs-resolved transmission electron microscope based on thermionic gun technology. Chemical Physics. 2013;**423**:79-84. DOI: 10.1016/j.chemphys.2013.06.026

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[6] Feist A, Bach N, Rubiano da Silva N, Danz T, Möller M, Priebe KE, et al. Ultrafast transmission electron microscopy using a laser-driven field emitter: Femtosecond resolution with a high coherence electron beam. Ultramicroscopy. 2017;**176**:63-73. DOI: 10.1016/j.ultramic.2016.12.005

[7] Kuwahara M, Nambo Y, Aoki K, Sameshima K, Jin X, Ujihara T, et al. The Boersch effect in a picosecond pulsed electron beam emitted from a semiconductor photocathode. Applied Physics Letters. 2016;**109**:013108. DOI: 10.1063/1.4955457

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[9] Manz S, Casandruc A, Zhang D, Zhong Y, Loch RA, Marx A, et al. Mapping atomic motions with ultrabright electrons: Towards fundamental limits in space-time resolution. Faraday Discussions. 2015;**177**:467-491. DOI: 10.1039/ C4FD00204K

[10] Yang J, Yoshida Y, Shibata H. Femtosecond time-resolved electron microscopy. Electronics and Communications in Japan. 2015;**98**: 50-57. DOI: 10.1002/ecj.11763

[11] Yang J, Yoshida Y, Yasuda H. Ultrafast electron microscopy with relativistic femtosecond electron pulses. Microscopy. 2018;**67**:291-295. DOI: 10.1093/jmicro/dfy032

[12] Yang J. Ultrafast electron microscopy with relativistic femtosecond electron pulses. In: Arita M, Sakaguchi N, editors. Electron Microscopy: Novel Microscopy Trends. Croatia: IntechOpen; 2019. DOI: 10.5772/intechopen.81405

[13] Yang J. Ultrafast electron microscopy reinventing femtosecond atomic scale imaging. Research OUTREACH. 2020;**112**:26-29. DOI: 10.32907/RO-112-2629. Available from: https://researchoutreach.org/ articles/ultrafast-electron-microscopyreinventingfemtosecond-atomic-scaleimaging/

[14] Yang J. New crystallography using relativistic femtosecond electron pulses. Impact. 2019;**10**:76-78. DOI: 10.21820/23987073.2019.10.76

[15] Yang J. Femtosecond electron diffraction with relativistic electron

**7**

*Introductory Chapter: 4D Imaging*

[16] Murooka Y, Naruse N,

DOI: 10.1063/1.3602314

10.1063/1.4847695

PhysRevB.88.184101

10.1155/2019/9739241

10.3390/qubs4010004

[20] Yang J, Gen K, Naruse N, Sakakihara S, Yoshida Y. A compact ultrafast electron diffractometer with relativistic femtosecond electron pulses. Quantum Beam Science. 2020;**4**:4. DOI:

Sakakihara S, Ishimaru M, Yang J, Tanimura K. Transmission-electron diffraction by MeV electron pulses. Applied Physics Letters. 2011;**98**:251903.

[17] Giret Y, Naruse N, Daraszewicz SL, Murooka Y, Yang J, Duffy DM, et al. Determination of transient atomic structure of laser-excited materials from time-resolved diffraction data. Applied Physics Letters. 2013;**103**:253107. DOI:

[18] Daraszewicz SL, Giret Y, Naruse N, Murooka Y, Yang J, Duffy DM, et al. Structural dynamics of laserirradiated gold nanofilms. Physical Review B. 2013;**88**:184101. DOI: 10.1103/

[19] Yang J, Yoshida Y. Relativistic ultrafast electron microscopy: Single-shot diffraction imaging with femtosecond electron pulses. Advances in Condensed Matter Physics. 2019;**2019**:9739241. DOI:

*DOI: http://dx.doi.org/10.5772/intechopen.92350*

pulses. In: Yang J, editor. Novel Imaging and Spectroscopy. Croatia: IntechOpen; 2020. DOI: 10.5772/intechopen.88511

*Introductory Chapter: 4D Imaging DOI: http://dx.doi.org/10.5772/intechopen.92350*

pulses. In: Yang J, editor. Novel Imaging and Spectroscopy. Croatia: IntechOpen; 2020. DOI: 10.5772/intechopen.88511

[16] Murooka Y, Naruse N, Sakakihara S, Ishimaru M, Yang J, Tanimura K. Transmission-electron diffraction by MeV electron pulses. Applied Physics Letters. 2011;**98**:251903. DOI: 10.1063/1.3602314

[17] Giret Y, Naruse N, Daraszewicz SL, Murooka Y, Yang J, Duffy DM, et al. Determination of transient atomic structure of laser-excited materials from time-resolved diffraction data. Applied Physics Letters. 2013;**103**:253107. DOI: 10.1063/1.4847695

[18] Daraszewicz SL, Giret Y, Naruse N, Murooka Y, Yang J, Duffy DM, et al. Structural dynamics of laserirradiated gold nanofilms. Physical Review B. 2013;**88**:184101. DOI: 10.1103/ PhysRevB.88.184101

[19] Yang J, Yoshida Y. Relativistic ultrafast electron microscopy: Single-shot diffraction imaging with femtosecond electron pulses. Advances in Condensed Matter Physics. 2019;**2019**:9739241. DOI: 10.1155/2019/9739241

[20] Yang J, Gen K, Naruse N, Sakakihara S, Yoshida Y. A compact ultrafast electron diffractometer with relativistic femtosecond electron pulses. Quantum Beam Science. 2020;**4**:4. DOI: 10.3390/qubs4010004

**6**

*Novel Imaging and Spectroscopy*

DOI: 10.1142/p641

**References**

science.1166135

pnas.0609233103

[1] Zewail AH, Thomas JM. 4D Electron Microscopy: Imaging in Space and Time. London: Imperial College Press; 2010.

a high brightness ultrafast transmission electron microscope based on a laserdriven cold field emission source. Ultramicroscopy. 2018;**186**:128-138. DOI: 10.1016/j.ultramic.2017.12.015

[9] Manz S, Casandruc A, Zhang D, Zhong Y, Loch RA, Marx A, et al. Mapping atomic motions with ultrabright electrons: Towards fundamental limits in space-time resolution. Faraday Discussions. 2015;**177**:467-491. DOI: 10.1039/

[10] Yang J, Yoshida Y, Shibata H. Femtosecond time-resolved electron

[11] Yang J, Yoshida Y, Yasuda H. Ultrafast electron microscopy with relativistic femtosecond electron pulses. Microscopy. 2018;**67**:291-295. DOI:

10.1093/jmicro/dfy032

[12] Yang J. Ultrafast electron microscopy with relativistic femtosecond electron pulses. In: Arita M, Sakaguchi N, editors. Electron Microscopy: Novel Microscopy Trends. Croatia: IntechOpen; 2019. DOI: 10.5772/intechopen.81405

[13] Yang J. Ultrafast electron microscopy reinventing femtosecond atomic scale imaging.

imaging/

Research OUTREACH. 2020;**112**:26-29. DOI: 10.32907/RO-112-2629. Available from: https://researchoutreach.org/ articles/ultrafast-electron-microscopyreinventingfemtosecond-atomic-scale-

[14] Yang J. New crystallography using relativistic femtosecond electron pulses. Impact. 2019;**10**:76-78. DOI: 10.21820/23987073.2019.10.76

[15] Yang J. Femtosecond electron diffraction with relativistic electron

microscopy. Electronics and Communications in Japan. 2015;**98**: 50-57. DOI: 10.1002/ecj.11763

C4FD00204K

[2] Zewail AH. Four-Dimensional Electron Microscopy. Science. 2010;**328**:187-193. DOI: 10.1126/

[3] Grinolds MS, Lobastov VA,

Weissenrieder J, Zewail AH. Proceedings of the National Academy of Sciences of the United States of America. 2006;**103**:18427-18431. DOI: 10.1073/

[4] Piazza L, Masiel DJ, LaGrange T, Reed BW, Barwick B, Carbone F. Design and implementation of a fs-resolved transmission electron microscope based on thermionic gun technology. Chemical Physics. 2013;**423**:79-84. DOI:

10.1016/j.chemphys.2013.06.026

[5] Bücker K, Picher M, Crégut O, LaGrange T, Reed BW, Park ST, et al. Electron beam dynamics in an ultrafast transmission electron microscope with Wehnelt electrode. Ultramicroscopy.

2016;**171**:8-18. DOI: 10.1016/j.

[6] Feist A, Bach N, Rubiano da Silva N, Danz T, Möller M, Priebe KE, et al. Ultrafast transmission electron microscopy using a laser-driven field emitter: Femtosecond resolution with a high coherence electron beam. Ultramicroscopy. 2017;**176**:63-73. DOI:

10.1016/j.ultramic.2016.12.005

10.1063/1.4955457

[7] Kuwahara M, Nambo Y, Aoki K, Sameshima K, Jin X, Ujihara T, et al. The Boersch effect in a picosecond pulsed electron beam emitted from a semiconductor photocathode. Applied Physics Letters. 2016;**109**:013108. DOI:

[8] Houdellier F, Caruso GM, Weber S, Kociak M, Arbouet A. Development of

ultramic.2016.08.014

**Chapter 2**

*Jinfeng Yang*

**1. Introduction**

**9**

**Abstract**

Femtosecond Electron Diffraction

Using Relativistic Electron Pulses

Observation of atomic-scale structural motion in matter with femtosecond temporal resolution is of considerable interest to scientists and paves the way for new science and applications. For this purpose, ultrafast electron diffraction (UED) imaging using femtosecond electron pulses is a very promising technique, as electrons have a larger elastic scattering cross section as compared to photons or X-rays and can be easily focused in observation with high spatial resolution. In this chapter, we first give an overview of the historical development of current nonrelativistic UEDs and discuss the potentials of UEDs with relativistic electron pulses. Second, we describe the concept and development of relativistic UED with femtosecond electron pulses generated by a radio-frequency acceleration-based photoemission gun. Some demonstrations of diffraction imaging of crystalline materials using 3- MeV electron pulses with durations of 100 fs are presented. Finally, we report a methodology of single-shot time-resolved diffraction imaging for the study of

ultrafast dynamics of photo-induced irreversible phase transitions.

**Keywords:** ultrafast electron diffraction, femtosecond electron pulse, relativistic electron beam, structural dynamics, radio-frequency electron gun

Femtosecond imaging is a long-awaited technique for materials scientists to observe atomic and molecular motions directly and in real time. For this type of ultrafast imaging, time-resolved diffraction with short-pulsed X-rays has been developed and is widely used globally. Recently, ultrafast electron diffraction (UED) using femtosecond electron pulses [1–3] has facilitated the study of the structural dynamics of reversible and irreversible processes, including ultrafast phase transformations, femtosecond chemical/biochemical reactions, and radiation damages. It is well-known that electrons have many advantages over X-rays. Electrons have a larger elastic scattering cross section and can be easily focused to develop a measurement with high spatial resolution. A small sample can be used in the UED measurement. The instruments used with UEDs are also very compact. The earliest time-resolved electron diffraction experiments with pulsed electrons on the milli- to microsecond time scale were developed using a deflection technique in the 1980s [4]. Later, a photoemission pulsed electron source was used in an electron diffraction measurement by Ewbank et al. [5, 6]. The temporal resolution was improved to sub-nanoseconds. In 1982, Mourou and Williamson [7] pioneered the use of 100-ps electron pulses to construct a picosecond electron

## **Chapter 2**

## Femtosecond Electron Diffraction Using Relativistic Electron Pulses

*Jinfeng Yang*

## **Abstract**

Observation of atomic-scale structural motion in matter with femtosecond temporal resolution is of considerable interest to scientists and paves the way for new science and applications. For this purpose, ultrafast electron diffraction (UED) imaging using femtosecond electron pulses is a very promising technique, as electrons have a larger elastic scattering cross section as compared to photons or X-rays and can be easily focused in observation with high spatial resolution. In this chapter, we first give an overview of the historical development of current nonrelativistic UEDs and discuss the potentials of UEDs with relativistic electron pulses. Second, we describe the concept and development of relativistic UED with femtosecond electron pulses generated by a radio-frequency acceleration-based photoemission gun. Some demonstrations of diffraction imaging of crystalline materials using 3- MeV electron pulses with durations of 100 fs are presented. Finally, we report a methodology of single-shot time-resolved diffraction imaging for the study of ultrafast dynamics of photo-induced irreversible phase transitions.

**Keywords:** ultrafast electron diffraction, femtosecond electron pulse, relativistic electron beam, structural dynamics, radio-frequency electron gun

## **1. Introduction**

Femtosecond imaging is a long-awaited technique for materials scientists to observe atomic and molecular motions directly and in real time. For this type of ultrafast imaging, time-resolved diffraction with short-pulsed X-rays has been developed and is widely used globally. Recently, ultrafast electron diffraction (UED) using femtosecond electron pulses [1–3] has facilitated the study of the structural dynamics of reversible and irreversible processes, including ultrafast phase transformations, femtosecond chemical/biochemical reactions, and radiation damages. It is well-known that electrons have many advantages over X-rays. Electrons have a larger elastic scattering cross section and can be easily focused to develop a measurement with high spatial resolution. A small sample can be used in the UED measurement. The instruments used with UEDs are also very compact.

The earliest time-resolved electron diffraction experiments with pulsed electrons on the milli- to microsecond time scale were developed using a deflection technique in the 1980s [4]. Later, a photoemission pulsed electron source was used in an electron diffraction measurement by Ewbank et al. [5, 6]. The temporal resolution was improved to sub-nanoseconds. In 1982, Mourou and Williamson [7] pioneered the use of 100-ps electron pulses to construct a picosecond electron

diffraction experiment. Later, they used a streak-camera tube to generate 25-keV picosecond electron pulses (�20 ps) in a UED experiment and succeeded at detecting laser-induced ultrafast phase transformation in aluminum [8]. The number of electrons per pulse used in the diffraction measurement was maintained at �10<sup>4</sup> . Since 2000, the research groups of Zewail [1, 9, 10], Miller [2], and Cao [11] have focused their efforts on generating ultrashort electron pulses. A modelocked femtosecond laser was used to generate ultrashort electron pulses and to excite the samples. A temporal resolution of sub-picoseconds and femtoseconds was achieved in UEDs, which provided real-time diffraction imaging and enabled the recording of atomic or molecular motion in chemical and biochemical reactions. **Figure 1** shows a schematic of a typical femtosecond electron-diffraction apparatus constructed with a laser-driven electron source, magnetic lens, sample-positioning system, and electron diffraction detector. In most UEDs, a conventional Ti:sapphire femtosecond pulsed laser is used to generate ultrashort electron pulses with a photocathode and to pump the sample to induce structural changes in materials. The key elements of this apparatus are the electron source and detection system.

a 30-keV and 300-fs electron pulse containing <sup>10</sup><sup>4</sup> electrons is transported to the sample with a 40-cm-long drift space, the pulse duration is increased to 4 ps,

temporal and spatial resolutions are reduced by the space-charge effects. To reduce the space-charge effects, two solutions are used in UED. One involves a decrease in the number of electrons in the pulse. This results in a low brightness electron beam at the sample and presents tremendous difficulties for single-shot imaging in the study of the dynamics of irreversible processes. Another involves reducing the photocathode-to-sample distance. Miller et al. at the University of Toronto developed a 30-keV electron gun with a photocathode-to-sample distance of 4.5 cm. This source produced a 600-fs short electron pulse containing 6000 electrons per pulse. The beam fluence at the sample *φ* = *N*/*A*, where N is the total

number of electrons per pulse and A is the beam area at the sample, is

of 100 fs, an advanced accelerator technology for radio-frequency (RF)

Therefore, the electrons emitted from the photocathode can be quickly

electron microscopy using the RF gun. They succeeded in generating high-brightness electron pulses with a pulse duration of 100 fs containing

and irreversible processes in materials can be observed.

pulse are needed to resolve the diffraction peak and/or obtain a clear image

To generate high-brightness electron beams with pulse durations on the order

acceleration-based photoemission electron guns (photocathode RF guns) has been proposed to generate multi-MeV femtosecond electron pulses for UED [16–27]. The RF gun is usually operated with a high RF electric field equal to or >100 MV/m.

accelerated into the relativistic energy region to minimize the space-charge effects in the pulse, yielding a femtosecond or picosecond pulse with numerous electrons. Recently, Yang et al. [28–30] developed the first prototype for relativistic ultrafast

10<sup>7</sup> electrons at an energy of 3 MeV. They also demonstrated the single-shot imaging using these femtosecond electron pulses [31, 32]. Relativistic UED is very promising for the study of ultrafast dynamics in solid-state materials and chemical/biological complex systems. It exhibits many crucial advantages over

• High-current electron pulses enable single-shot imaging so that both reversible

• Higher energies considerably enhance the extinction distance for elastic scattering and provide structural information that is essentially free from multiple scattering and inelastic effects. This enables us to easily understand

• Ultrashort pulses of <100 fs are possible. The utilization of the relativistic electron pulse overcomes the loss of temporal resolution because of the velocity

mismatch in samples. High temporal resolution of 100 fs or less can be

• A thick sample can be used for measurement, thus obviating the requirement

• Radiation effects derived from ionization damage processes decrease at higher energies in samples. In fact, at MeV energies, the dominant damage process for electron beams is not ionization but rather the much slower ballistic "knock-

*<sup>φ</sup>* <sup>1</sup> <sup>10</sup><sup>11</sup> <sup>m</sup><sup>2</sup> [14]. Nevertheless, a minimum of <sup>10</sup><sup>4</sup>

in UED [15].

nonrelativistic UED systems:

and explain structural dynamics.

achieved in relativistic UED.

on" process.

**11**

to prepare suitable thin samples.

. In this case, both the

–10<sup>5</sup> electrons per

and the relative energy spread is increased to 3 <sup>10</sup><sup>3</sup>

*Femtosecond Electron Diffraction Using Relativistic Electron Pulses*

*DOI: http://dx.doi.org/10.5772/intechopen.88511*

The temporal resolution is mainly defined by the electron pulse duration. In all of the aforementioned UEDs, a DC acceleration-based photoemission electron gun was used to generate ultrashort electron pulses with a short-pulsed laser. The use of a higher accelerating electric field (extraction field, *E*ext) between the photocathode and anode (grid) is crucial to reducing Coulomb repulsion of electrons (spacecharge effects). The transit-time broadening *τ*KE can be reduced by increasing *E*ext, because *τKE* ∝ ffiffiffiffiffiffiffiffiffiffiffi <sup>Δ</sup>*Ekin* <sup>p</sup> *<sup>=</sup>Eext* [12], where <sup>Δ</sup>*Ekin* is the kinetic energy spread of photoelectrons. For an Au or Ag photocathode driven by a 266-nm ultraviolet (UV) laser, Δ*Ekin* � 0.6 eV, the dispersive broadening near the photocathode can be maintained below 300 fs at *E*ext = 10 MV/m or more [3]. However, the maximum static extraction field in the dc gun is determined by the vacuum breakdown limit of �10 MV/m.

The space-charge effects occur not only in the electron gun but also during the propagation of the electron pulse from the gun to the sample. The broadening of both the pulse duration and energy spread of electrons due to the space-charge effects in the propagation of nonrelativistic femtosecond electron pulses has been investigated theoretically by Siwick et al. [13]. Their results indicated that when

#### **Figure 1.**

*Schematic of a typical femtosecond electron-diffraction apparatus. A DC-acceleration-based photoemission electron gun is used to produce ultrashort electron pulses with third harmonics of a Ti:sapphire femtosecond laser. The electron pulse passes through the specimen and produces a diffraction pattern of the structure.*

*Femtosecond Electron Diffraction Using Relativistic Electron Pulses DOI: http://dx.doi.org/10.5772/intechopen.88511*

diffraction experiment. Later, they used a streak-camera tube to generate 25-keV picosecond electron pulses (�20 ps) in a UED experiment and succeeded at detecting laser-induced ultrafast phase transformation in aluminum [8]. The number of electrons per pulse used in the diffraction measurement was maintained

[11] have focused their efforts on generating ultrashort electron pulses. A modelocked femtosecond laser was used to generate ultrashort electron pulses and to excite the samples. A temporal resolution of sub-picoseconds and femtoseconds was achieved in UEDs, which provided real-time diffraction imaging and enabled the recording of atomic or molecular motion in chemical and biochemical reactions. **Figure 1** shows a schematic of a typical femtosecond electron-diffraction apparatus constructed with a laser-driven electron source, magnetic lens, sample-positioning system, and electron diffraction detector. In most UEDs, a conventional Ti:sapphire femtosecond pulsed laser is used to generate ultrashort electron pulses with a photocathode and to pump the sample to induce structural changes in materials. The key elements of this apparatus are the electron source and detection system. The temporal resolution is mainly defined by the electron pulse duration. In all of the aforementioned UEDs, a DC acceleration-based photoemission electron gun was used to generate ultrashort electron pulses with a short-pulsed laser. The use of a higher accelerating electric field (extraction field, *E*ext) between the photocathode and anode (grid) is crucial to reducing Coulomb repulsion of electrons (spacecharge effects). The transit-time broadening *τ*KE can be reduced by increasing *E*ext,

. Since 2000, the research groups of Zewail [1, 9, 10], Miller [2], and Cao

<sup>p</sup> *<sup>=</sup>Eext* [12], where <sup>Δ</sup>*Ekin* is the kinetic energy spread of photo-

electrons. For an Au or Ag photocathode driven by a 266-nm ultraviolet (UV) laser, Δ*Ekin* � 0.6 eV, the dispersive broadening near the photocathode can be maintained

The space-charge effects occur not only in the electron gun but also during the propagation of the electron pulse from the gun to the sample. The broadening of both the pulse duration and energy spread of electrons due to the space-charge effects in the propagation of nonrelativistic femtosecond electron pulses has been investigated theoretically by Siwick et al. [13]. Their results indicated that when

*Schematic of a typical femtosecond electron-diffraction apparatus. A DC-acceleration-based photoemission electron gun is used to produce ultrashort electron pulses with third harmonics of a Ti:sapphire femtosecond laser. The electron pulse passes through the specimen and produces a diffraction pattern of the structure.*

below 300 fs at *E*ext = 10 MV/m or more [3]. However, the maximum static extraction field in the dc gun is determined by the vacuum breakdown limit of

at �10<sup>4</sup>

*Novel Imaging and Spectroscopy*

because *τKE* ∝ ffiffiffiffiffiffiffiffiffiffiffi

�10 MV/m.

**Figure 1.**

**10**

Δ*Ekin*

a 30-keV and 300-fs electron pulse containing <sup>10</sup><sup>4</sup> electrons is transported to the sample with a 40-cm-long drift space, the pulse duration is increased to 4 ps, and the relative energy spread is increased to 3 <sup>10</sup><sup>3</sup> . In this case, both the temporal and spatial resolutions are reduced by the space-charge effects. To reduce the space-charge effects, two solutions are used in UED. One involves a decrease in the number of electrons in the pulse. This results in a low brightness electron beam at the sample and presents tremendous difficulties for single-shot imaging in the study of the dynamics of irreversible processes. Another involves reducing the photocathode-to-sample distance. Miller et al. at the University of Toronto developed a 30-keV electron gun with a photocathode-to-sample distance of 4.5 cm. This source produced a 600-fs short electron pulse containing 6000 electrons per pulse. The beam fluence at the sample *φ* = *N*/*A*, where N is the total number of electrons per pulse and A is the beam area at the sample, is *<sup>φ</sup>* <sup>1</sup> <sup>10</sup><sup>11</sup> <sup>m</sup><sup>2</sup> [14]. Nevertheless, a minimum of <sup>10</sup><sup>4</sup> –10<sup>5</sup> electrons per pulse are needed to resolve the diffraction peak and/or obtain a clear image in UED [15].

To generate high-brightness electron beams with pulse durations on the order of 100 fs, an advanced accelerator technology for radio-frequency (RF) acceleration-based photoemission electron guns (photocathode RF guns) has been proposed to generate multi-MeV femtosecond electron pulses for UED [16–27]. The RF gun is usually operated with a high RF electric field equal to or >100 MV/m. Therefore, the electrons emitted from the photocathode can be quickly accelerated into the relativistic energy region to minimize the space-charge effects in the pulse, yielding a femtosecond or picosecond pulse with numerous electrons. Recently, Yang et al. [28–30] developed the first prototype for relativistic ultrafast electron microscopy using the RF gun. They succeeded in generating high-brightness electron pulses with a pulse duration of 100 fs containing 10<sup>7</sup> electrons at an energy of 3 MeV. They also demonstrated the single-shot imaging using these femtosecond electron pulses [31, 32]. Relativistic UED is very promising for the study of ultrafast dynamics in solid-state materials and chemical/biological complex systems. It exhibits many crucial advantages over nonrelativistic UED systems:


• Relativistic UED is suitable for in situ observations, as large areas exist in the sample room for installing various specimens. Relativistic UED can be used to study gas-, liquid-, and solid-phase samples.

**2.1 Photocathode RF gun**

*DOI: http://dx.doi.org/10.5772/intechopen.88511*

The photocathode RF gun is a high-brightness electron source and has been widely applied in the field of advanced particle accelerators (e.g., free-electron lasers and linear colliders). The RF gun used in relativistic UED consists of two RF cavities: a half cell and a full cell (1.6-cell), as shown in **Figure 3**. The length of the full-cell cavity is equal to half the 2.856 GHz RF wavelength, *λ*/2 = 52.48 mm, whereas the length of the half cell is 0.6 times *λ*/2; numerical studies have shown that an optimal performance is obtained if the half-cell cavity is 0.6 times the full cell length rather than 0.5�. The design and details of the cavities were described in [32, 33]. The cavities are driven by a MW-power 2.856 GHz accelerating RF to produce a stronger accelerating RF field on the photocathode. The RF gun is operated in the TM010 transverse magnetic mode [34, 35]. The phase shift between the half and full cells is equal to π, resulting in the acceleration of electrons in both cavities. The linear components of the electric fields

where *E*<sup>0</sup> is the peak accelerating field, *k* = 2*π*/*λ*, *ω* = *ck*, *c* is the velocity of light, and *ϕ*<sup>0</sup> is the initial RF phase when the electron leaves the cathode surface (*z* = 0) at *t* = 0. In relativistic UED, the peak RF power filled to the RF cavities is 4 MW, resulting in *E*<sup>0</sup> = 75 MV/m. This field thus accelerates electrons emitted from the photocathode quickly up to 3 MeV to minimize the space-charge effects in the pulses. Therefore, the RF gun can easily produce femtosecond electron pulses by the

The visualization of atomic-scale structural motion by UED requires electron pulses of the shortest duration and lowest emittance to achieve high temporal and spatial coherence. The temporal resolution of UED is determined based on the duration of the electron pulses. The beam emittance directly determines the quality of the diffraction image (e.g., the sharpness of the diffraction patterns (DPs) and the diffraction contrast in the acquired images (i.e., spatial resolution)). The pulse duration and emittance of the electron beam are two crucial parameters in UED.

The temporal duration of the electron pulses generated from the RF gun, in the absence of time delay due to the response time of the photocathode materials, is

*(a) Photocathode RF gun and (b) schematic for the generation of femtosecond electron pulses in the RF gun.*

*Ez* ¼ *E*<sup>0</sup> cos *kz* sin *ωt* þ *φ*<sup>0</sup> ð Þ*,* (1)

at *r* = 0 (the center axis of the RF cavities) [32] can be assumed to be:

*Femtosecond Electron Diffraction Using Relativistic Electron Pulses*

illumination of the femtosecond laser on the photocathode.

*2.1.1 The temporal duration of the electron pulses*

**Figure 3.**

**13**

In this chapter, we introduce UED with relativistic femtosecond electron pulses. The chapter also describes the generation of femtosecond electron pulses using an RF photoemission gun, the concept and design of relativistic UED, and demonstration experiments with relativistic femtosecond electron pulses.

## **2. Relativistic UED with femtosecond electron pulses**

Relativistic UED consists of a 1.6-cell S-band (2.856 GHz) photocathode RF gun, an imaging system with magnetic lenses, a femtosecond laser, and a detector. **Figure 2** shows a schematic and photograph of relativistic UED at Osaka University. All components were installed on a vibration-controlled board with a size of <sup>3</sup> 3 m<sup>2</sup> .

*Schematic (top) and photograph (bottom) of UED apparatus with relativistic femtosecond electron pulses at Osaka University. All components are installed on a vibration-controlled board with a size of 3 3 m<sup>2</sup> .*

## **2.1 Photocathode RF gun**

• Relativistic UED is suitable for in situ observations, as large areas exist in the sample room for installing various specimens. Relativistic UED can be used to

Relativistic UED consists of a 1.6-cell S-band (2.856 GHz) photocathode RF gun,

an imaging system with magnetic lenses, a femtosecond laser, and a detector. **Figure 2** shows a schematic and photograph of relativistic UED at Osaka University. All components were installed on a vibration-controlled board with a size of

*Schematic (top) and photograph (bottom) of UED apparatus with relativistic femtosecond electron pulses at Osaka University. All components are installed on a vibration-controlled board with a size of 3 3 m<sup>2</sup>*

*.*

In this chapter, we introduce UED with relativistic femtosecond electron pulses. The chapter also describes the generation of femtosecond electron pulses using an RF photoemission gun, the concept and design of relativistic UED, and demonstration experiments with relativistic femtosecond electron pulses.

study gas-, liquid-, and solid-phase samples.

**2. Relativistic UED with femtosecond electron pulses**

<sup>3</sup> 3 m<sup>2</sup>

**Figure 2.**

**12**

.

*Novel Imaging and Spectroscopy*

The photocathode RF gun is a high-brightness electron source and has been widely applied in the field of advanced particle accelerators (e.g., free-electron lasers and linear colliders). The RF gun used in relativistic UED consists of two RF cavities: a half cell and a full cell (1.6-cell), as shown in **Figure 3**. The length of the full-cell cavity is equal to half the 2.856 GHz RF wavelength, *λ*/2 = 52.48 mm, whereas the length of the half cell is 0.6 times *λ*/2; numerical studies have shown that an optimal performance is obtained if the half-cell cavity is 0.6 times the full cell length rather than 0.5�. The design and details of the cavities were described in [32, 33]. The cavities are driven by a MW-power 2.856 GHz accelerating RF to produce a stronger accelerating RF field on the photocathode. The RF gun is operated in the TM010 transverse magnetic mode [34, 35]. The phase shift between the half and full cells is equal to π, resulting in the acceleration of electrons in both cavities. The linear components of the electric fields at *r* = 0 (the center axis of the RF cavities) [32] can be assumed to be:

$$E\_x = E\_0 \cos kz \sin \left(at + \varphi\_0\right),\tag{1}$$

where *E*<sup>0</sup> is the peak accelerating field, *k* = 2*π*/*λ*, *ω* = *ck*, *c* is the velocity of light, and *ϕ*<sup>0</sup> is the initial RF phase when the electron leaves the cathode surface (*z* = 0) at *t* = 0. In relativistic UED, the peak RF power filled to the RF cavities is 4 MW, resulting in *E*<sup>0</sup> = 75 MV/m. This field thus accelerates electrons emitted from the photocathode quickly up to 3 MeV to minimize the space-charge effects in the pulses. Therefore, the RF gun can easily produce femtosecond electron pulses by the illumination of the femtosecond laser on the photocathode.

The visualization of atomic-scale structural motion by UED requires electron pulses of the shortest duration and lowest emittance to achieve high temporal and spatial coherence. The temporal resolution of UED is determined based on the duration of the electron pulses. The beam emittance directly determines the quality of the diffraction image (e.g., the sharpness of the diffraction patterns (DPs) and the diffraction contrast in the acquired images (i.e., spatial resolution)). The pulse duration and emittance of the electron beam are two crucial parameters in UED.

#### *2.1.1 The temporal duration of the electron pulses*

The temporal duration of the electron pulses generated from the RF gun, in the absence of time delay due to the response time of the photocathode materials, is

### **Figure 3.**

*(a) Photocathode RF gun and (b) schematic for the generation of femtosecond electron pulses in the RF gun.*

given by the driving laser pulse duration and temporal electron broadening. The temporal electron broadening can be defined with two components: one derives from the initial energy bandwidth (initial kinetic energy spread, Δ*Ekin*) of electrons emitted from the photocathode, as described in Section 1. Another derives from the space-charge-induced broadening during the propagation from the cathode to the sample. Therefore, the electron pulse duration can be given as

$$
\sigma\_b = \sqrt{\sigma\_{opt}^2 + \tau\_{KE}^2 + \tau\_{SC}^2} \tag{2}
$$

emittance is estimated as a function of the RMS laser spot size: *εth* = 0.74 � *σr*. This indicates that we can reduce the emittance to *εth* � 10 nm-rad in the RF gun if we can focus the laser spot as *σ<sup>r</sup>* = 10 μm at the photocathode. In this case, the peak

*Bp* <sup>¼</sup> ð Þ *βγ* <sup>2</sup> *<sup>Q</sup>*

where *β* = *v/c*, *v* is the electron velocity, and *γ* is the normalized relativistic energy. From a 3-MeV electron pulse with a pulse duration of 100 fs, we can calculate the peak brightness to *Bp* = 5 � <sup>10</sup><sup>17</sup> A/m<sup>2</sup> sr and the beam fluence to

> *Lc* <sup>¼</sup> *<sup>h</sup> m*0*c σx*

length is *Lc* � 10 nm, which is an ideal value for electron diffraction imaging. It is twice as large or greater than that of current nonrelativistic UED systems [12, 37, 38]. This allows us to detect sharp DPs and acquire good contrast diffraction

*ε*2*σ<sup>b</sup>*

. The spatial coherence length can be calculated by [37]:

where *h* is Planck's constant. If *σ<sup>x</sup>* = 0.3 mm at the sample, the spatial coherence

In the presented relativistic UED apparatus, a very fine copper photocathode was used and illuminated by the third-harmonic of a Ti:sapphire laser (266 nm, pulse duration: 90 fs). The pulse energy of the UV light was 5 μJ at maximum. The diameter of the laser spot at the photocathode was 0.1 mm in RMS focused by an optical lens. The injection phase (gun phase) was 30°, which is an optimal condition to minimize the transverse emittance. The electron beam energy was 3 MeV under the 4-MW RF input. The repetition rate of the electron pulses was 10 Hz, which

The electron illumination system consists of a solenoid magnetic lens, condenser lens, and condenser aperture to control and transfer the electron pulses from the RF gun on the specimen, as shown in **Figure 3**. The solenoid lens with a large beam aperture is used to create a parallel electron beam. The condenser aperture made of a 1-mm-thick molybdenum metal with four pinholes with diameters of 0.3, 0.5, 1, and 2 mm stops the large-divergence electrons to further reduce the emittance, yielding a small illumination convergence angle at the specimen. After the aperture, we use the condenser lens to create a parallel beam or convergent beam on the specimen. The parallel beam is used for selected area diffraction, whereas the convergent beam is used mainly for convergent beam electron diffraction.

The emittance of the electron beam that passed through the aperture with 0.5, 1

and 2 mm diameter pinholes was measured as 0.1, 0.3, and 0.7 mm-mrad [30], respectively. Reducing the emittance increased the RMS brightness in the pulse. The RMS brightness of the transmitted electrons was 2.2, 1.4 and 0.5 � 1022 electrons/ <sup>m</sup><sup>2</sup> sr, and the number of electrons per pulse was �0.6, 2.5, and 4.4 � <sup>10</sup><sup>7</sup> at the sample with 0.5-, 1-, and 2-mm diameter pinholes, respectively. For the use of the 0.3-mm-diameter pinhole, the number of electrons in the pulse was �<sup>1</sup> � <sup>10</sup><sup>6</sup>

and the brightness was estimated to be <sup>≥</sup><sup>5</sup> � <sup>10</sup><sup>22</sup> electrons/m<sup>2</sup> sr. The illumination convergence angle of the electron beam at the sample was *α* = 26 μrad in the

*,* (7)

*<sup>ε</sup> ,* (8)

,

brightness of electron pulses *Bp* can be calculated by

*Femtosecond Electron Diffraction Using Relativistic Electron Pulses*

*DOI: http://dx.doi.org/10.5772/intechopen.88511*

was determined by the repetition rate of the RF pulses.

*<sup>φ</sup>* � <sup>2</sup> � <sup>10</sup><sup>11</sup> <sup>m</sup>�<sup>2</sup>

images in the measurements.

**2.2 UED imaging system**

**15**

*2.2.1 Electron illumination system*

where *σ<sup>b</sup>* is the pulse duration of electrons, *σopt* is the laser pulse duration, and τKE and *τ*SC are the temporal broadenings due to the initial kinetic energy spread and space-charge effect, respectively. The first temporal broadening is proportional to *τKE* ∝ ffiffiffiffiffiffiffiffiffiffiffi <sup>Δ</sup>*Ekin* <sup>p</sup> *<sup>=</sup>Eacc* [12], where *Eacc* is the accelerating electric field (*E*ext in the dc gun and *E*<sup>0</sup> for the RF gun). The second temporal broadening is proportional to *Q*/*E*<sup>2</sup> , where *Q* and *E* are the electron charge and total energy of the electron beam, respectively. For the 3-MeV electron pulses at *Q* < 1 pC, *τ*KE and *τsc* in the RF gun are negligible. Therefore, the duration of low-charge electron pulses generated from the RF gun is approximately equal to the driving laser pulse duration, *σ<sup>b</sup>* ≈ *σopt*.

#### *2.1.2 The total emittance of the electron beam*

The total emittance of the electron beam generated from the RF gun [32] is given as:

$$
\sigma = \sqrt{\varepsilon\_{\eta^\sharp}^2 + \varepsilon\_{sc}^2 + \varepsilon\_{th^\*}^2} \tag{3}
$$

$$
\varepsilon\_{\rm ff} = 2.73 \times 10^{-11} E\_0 f^2 \sigma\_\chi^2 \sigma\_{b^\*}^2 \tag{4}
$$

*<sup>ε</sup>sc* <sup>¼</sup> <sup>3</sup>*:*<sup>76</sup> � <sup>10</sup><sup>3</sup> *<sup>Q</sup> E*0ð Þ 2*σ<sup>x</sup>* þ *σ<sup>b</sup> ,* (5)

where *εrf* is the normalized RF-induced emittance in root mean square (RMS) in mm-mrad, *εsc* is the normalized RMS space-charge-induced emittance in mm-mrad, *εth* is the thermal emittance (initial emittance) at the cathode, *E*<sup>0</sup> is the peak accelerating field in MV/m, *f* is the RF in MHz, *Q* is the electron charge of the pulse in nC, and *σ<sup>b</sup>* and *σ<sup>x</sup>* are the RMS pulse duration in ps and RMS transverse beam size in mm, respectively. For example, under the conditions of *E*<sup>0</sup> = 75 MV/m, *σ<sup>b</sup>* = 100 fs, *σ<sup>x</sup>* = 0.3 mm, and *Q* = 0.1 pC (10<sup>6</sup> electrons per pulse), we estimate that *<sup>ε</sup>rf* = 1.5 � <sup>10</sup>�<sup>5</sup> mm-mrad and *<sup>ε</sup>sc* = 7 � <sup>10</sup>�<sup>3</sup> mm-mrad. This indicates that the RFinduced emittance is negligible and the magnitude of the space-charge-induced emittance at *Q* ≤ 0.1 pC is close to the order of nm-rad. Therefore, in this case, the thermal emittance (initial emittance) at the cathode is dominant.

Assuming an isotropic emission into a half sphere in front of the cathode surface, the thermal emittance can be expressed in terms of the RMS incident laser spot size on the cathode *σ<sup>r</sup>* and the initial kinetic energy spread *ΔEkin* of the photoelectrons:

$$
\varepsilon\_{th} = \sigma\_r \sqrt{\frac{2\Delta E\_{kin}}{3m\_0 c^2}},\tag{6}
$$

where *m*<sup>0</sup> is the electron mass and *σ<sup>r</sup>* is the laser spot size at the photocathode. For a very fine copper photocathode driven by a 266-nm laser under the conditions of *E*<sup>0</sup> = 75 MV/m and *ϕ*<sup>0</sup> = 30°, Δ*Ekin* = 0.42 eV [36]. Therefore, the thermal

*Femtosecond Electron Diffraction Using Relativistic Electron Pulses DOI: http://dx.doi.org/10.5772/intechopen.88511*

emittance is estimated as a function of the RMS laser spot size: *εth* = 0.74 � *σr*. This indicates that we can reduce the emittance to *εth* � 10 nm-rad in the RF gun if we can focus the laser spot as *σ<sup>r</sup>* = 10 μm at the photocathode. In this case, the peak brightness of electron pulses *Bp* can be calculated by

$$B\_p = \left(\beta\gamma\right)^2 \frac{Q}{\varepsilon^2 \sigma\_b},\tag{7}$$

where *β* = *v/c*, *v* is the electron velocity, and *γ* is the normalized relativistic energy. From a 3-MeV electron pulse with a pulse duration of 100 fs, we can calculate the peak brightness to *Bp* = 5 � <sup>10</sup><sup>17</sup> A/m<sup>2</sup> sr and the beam fluence to *<sup>φ</sup>* � <sup>2</sup> � <sup>10</sup><sup>11</sup> <sup>m</sup>�<sup>2</sup> . The spatial coherence length can be calculated by [37]:

$$L\_{\varepsilon} = \frac{h}{m\_0 \varepsilon} \frac{\sigma\_{\chi}}{\varepsilon},\tag{8}$$

where *h* is Planck's constant. If *σ<sup>x</sup>* = 0.3 mm at the sample, the spatial coherence length is *Lc* � 10 nm, which is an ideal value for electron diffraction imaging. It is twice as large or greater than that of current nonrelativistic UED systems [12, 37, 38]. This allows us to detect sharp DPs and acquire good contrast diffraction images in the measurements.

In the presented relativistic UED apparatus, a very fine copper photocathode was used and illuminated by the third-harmonic of a Ti:sapphire laser (266 nm, pulse duration: 90 fs). The pulse energy of the UV light was 5 μJ at maximum. The diameter of the laser spot at the photocathode was 0.1 mm in RMS focused by an optical lens. The injection phase (gun phase) was 30°, which is an optimal condition to minimize the transverse emittance. The electron beam energy was 3 MeV under the 4-MW RF input. The repetition rate of the electron pulses was 10 Hz, which was determined by the repetition rate of the RF pulses.

#### **2.2 UED imaging system**

given by the driving laser pulse duration and temporal electron broadening. The temporal electron broadening can be defined with two components: one derives from the initial energy bandwidth (initial kinetic energy spread, Δ*Ekin*) of electrons emitted from the photocathode, as described in Section 1. Another derives from the space-charge-induced broadening during the propagation from the cathode to

> *σ*2 *opt* þ *τ*<sup>2</sup>

and *τ*SC are the temporal broadenings due to the initial kinetic energy spread and space-charge effect, respectively. The first temporal broadening is proportional to

and *E*<sup>0</sup> for the RF gun). The second temporal broadening is proportional to *Q*/*E*<sup>2</sup>

respectively. For the 3-MeV electron pulses at *Q* < 1 pC, *τ*KE and *τsc* in the RF gun are negligible. Therefore, the duration of low-charge electron pulses generated from the

The total emittance of the electron beam generated from the RF gun [32] is

*ε*2 *rf* þ *ε*<sup>2</sup>

*<sup>ε</sup>rf* <sup>¼</sup> <sup>2</sup>*:*<sup>73</sup> � <sup>10</sup>�<sup>11</sup>*E*0*<sup>f</sup>*

*<sup>ε</sup>sc* <sup>¼</sup> <sup>3</sup>*:*<sup>76</sup> � <sup>10</sup><sup>3</sup> *<sup>Q</sup>*

*σ<sup>x</sup>* = 0.3 mm, and *Q* = 0.1 pC (10<sup>6</sup> electrons per pulse), we estimate that

*εth* ¼ *σ<sup>r</sup>*

of *E*<sup>0</sup> = 75 MV/m and *ϕ*<sup>0</sup> = 30°, Δ*Ekin* = 0.42 eV [36]. Therefore, the thermal

thermal emittance (initial emittance) at the cathode is dominant.

q

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

where *εrf* is the normalized RF-induced emittance in root mean square (RMS) in mm-mrad, *εsc* is the normalized RMS space-charge-induced emittance in mm-mrad, *εth* is the thermal emittance (initial emittance) at the cathode, *E*<sup>0</sup> is the peak accelerating field in MV/m, *f* is the RF in MHz, *Q* is the electron charge of the pulse in nC, and *σ<sup>b</sup>* and *σ<sup>x</sup>* are the RMS pulse duration in ps and RMS transverse beam size in mm, respectively. For example, under the conditions of *E*<sup>0</sup> = 75 MV/m, *σ<sup>b</sup>* = 100 fs,

*<sup>ε</sup>rf* = 1.5 � <sup>10</sup>�<sup>5</sup> mm-mrad and *<sup>ε</sup>sc* = 7 � <sup>10</sup>�<sup>3</sup> mm-mrad. This indicates that the RFinduced emittance is negligible and the magnitude of the space-charge-induced emittance at *Q* ≤ 0.1 pC is close to the order of nm-rad. Therefore, in this case, the

Assuming an isotropic emission into a half sphere in front of the cathode surface, the thermal emittance can be expressed in terms of the RMS incident laser spot size on the cathode *σ<sup>r</sup>* and the initial kinetic energy spread *ΔEkin* of the photoelectrons:

r

where *m*<sup>0</sup> is the electron mass and *σ<sup>r</sup>* is the laser spot size at the photocathode. For a very fine copper photocathode driven by a 266-nm laser under the conditions

ffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2Δ*Ekin* 3*m*0*c*<sup>2</sup>

*sc* þ *ε*<sup>2</sup> *th*

> 2 *σ*2 *xσ*2

*E*0ð Þ 2*σ<sup>x</sup>* þ *σ<sup>b</sup>*

where *Q* and *E* are the electron charge and total energy of the electron beam,

RF gun is approximately equal to the driving laser pulse duration, *σ<sup>b</sup>* ≈ *σopt*.

*ε* ¼

*2.1.2 The total emittance of the electron beam*

q

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

where *σ<sup>b</sup>* is the pulse duration of electrons, *σopt* is the laser pulse duration, and τKE

<sup>p</sup> *<sup>=</sup>Eacc* [12], where *Eacc* is the accelerating electric field (*E*ext in the dc gun

*KE* þ *τ*<sup>2</sup> *SC*

*,* (2)

*,* (3)

*,* (6)

*b,* (4)

*,* (5)

,

the sample. Therefore, the electron pulse duration can be given as

*σ<sup>b</sup>* ¼

*τKE* ∝ ffiffiffiffiffiffiffiffiffiffiffi Δ*Ekin*

*Novel Imaging and Spectroscopy*

given as:

**14**

### *2.2.1 Electron illumination system*

The electron illumination system consists of a solenoid magnetic lens, condenser lens, and condenser aperture to control and transfer the electron pulses from the RF gun on the specimen, as shown in **Figure 3**. The solenoid lens with a large beam aperture is used to create a parallel electron beam. The condenser aperture made of a 1-mm-thick molybdenum metal with four pinholes with diameters of 0.3, 0.5, 1, and 2 mm stops the large-divergence electrons to further reduce the emittance, yielding a small illumination convergence angle at the specimen. After the aperture, we use the condenser lens to create a parallel beam or convergent beam on the specimen. The parallel beam is used for selected area diffraction, whereas the convergent beam is used mainly for convergent beam electron diffraction.

The emittance of the electron beam that passed through the aperture with 0.5, 1 and 2 mm diameter pinholes was measured as 0.1, 0.3, and 0.7 mm-mrad [30], respectively. Reducing the emittance increased the RMS brightness in the pulse. The RMS brightness of the transmitted electrons was 2.2, 1.4 and 0.5 � 1022 electrons/ <sup>m</sup><sup>2</sup> sr, and the number of electrons per pulse was �0.6, 2.5, and 4.4 � <sup>10</sup><sup>7</sup> at the sample with 0.5-, 1-, and 2-mm diameter pinholes, respectively. For the use of the 0.3-mm-diameter pinhole, the number of electrons in the pulse was �<sup>1</sup> � <sup>10</sup><sup>6</sup> , and the brightness was estimated to be <sup>≥</sup><sup>5</sup> � <sup>10</sup><sup>22</sup> electrons/m<sup>2</sup> sr. The illumination convergence angle of the electron beam at the sample was *α* = 26 μrad in the

parallel-beam operation mode with the 0.3-mm-diameter condenser aperture, which is discussed in a later section.

The specimen room is located downstream of the condenser lens. The distance from the photocathode to the specimen is 1.2 m. The sample is manipulated by sixaxis motorized stages. In a time-resolved experiment, the sample can be pumped by a femtosecond laser pulse, as shown in **Figure 2**. A pump laser with wavelengths of 266, 400, and 800 nm can be selected to meet the requirements of the measured materials. The pulse duration of the pump laser pulses is 90 fs in full width at half maximum (FWHM). The vacuum pressure in the specimen room reaches <sup>10</sup><sup>10</sup> Torr. When inserting or changing a new sample, the sample is first installed into a separated vacuum chamber (preparation room) for the sample cleaning.

**2.3 Femtosecond laser system**

*DOI: http://dx.doi.org/10.5772/intechopen.88511*

*Femtosecond Electron Diffraction Using Relativistic Electron Pulses*

compression.

rate of the electron pulses.

**compounds**

**17**

tively. The following four samples were used:

4.Multilayer single-crystalline mica films

A conventional mode-locked Ti:sapphire femtosecond laser (Spectra-Physics) is used to illuminate the photocathode and to excite the sample. The laser consists of a 80-fs Ti:sapphire laser oscillator (Tsunami, central wavelength: 800 nm) and a regenerative amplifier (Spitfire Ace) that includes a pulse stretcher and compressor. The femtosecond laser oscillator is synchronized to an external 79.3-MHz RF signal with a time-to-lock piezoelectric device, as shown in **Figure 2**. The 79.3-MHz RF signal is generated by dividing the accelerating 2856-MHz RF by 1/36. The time jitter between the laser pulse and RF phase is <100 fs. The laser oscillator output is fed to the regenerative amplifier for pulse stretching, amplification, and compression. The regenerative amplifier is driven by a green laser with a repetition rate of 1 kHz (Empower, wavelength: 532 nm, output: 20 W). The pulse energy of the amplifier output is 3 mJ. The pulse duration is 90 fs in FWHM after the pulse

The amplified femtosecond laser beam is divided into two beams. One is converted to the third-harmonics by a wavelength converter (Tripler) composed of two nonlinear crystals (SHG and THG) and a time plate for pulse delay adjustment. The third-harmonic pulses (UV wavelength, 266 nm; pulse duration, 90 fs) with a maximum energy of 5 μJ per pulse are focused by an optical lens and then illuminated onto the copper photocathode to generate femtosecond electron pulses. The residual fundamental femtosecond laser (wavelength: 800 nm) is used directly to excite the sample or is converted to second-harmonics (wavelength: 400 nm) or third-harmonics (wavelength: 266 nm) to excite the sample, based on the sample's requirements. The time delay between the pump laser pulse and the probe electron pulse is changed with an optical delay located on the pump laser beam line for time-resolved experiments, as shown in **Figure 2**. The repetition rate of the pump laser pulses is reduced to 10 Hz with two optical choppers, similar to the repetition

**3. UED experiments with relativistic femtosecond electron pulses**

**3.1 Observations of DPs from crystalline metals, semiconductors, and chemical**

In relativistic UED, we measured the DPs of various crystalline materials (e.g., metals, semiconductors, and chemical compounds). The electron pulses generated from the RF gun were collimated by the condenser aperture with the different pinhole diameters of 1, 0.5, and 0.3 mm before the sample was illuminated. The energy and pulse durations of the electron pulses were 3 MeV and 100 fs, respec-

1.35-nm-thick single-crystalline silicon (Si) films produced from a 60-μm-thick

3.Polycrystals of a thallous chloride chemical compound dispersed on a carbon

2.30-nm-thick polycrystalline aluminum foils (Cat. No. S108, EM-Japan)

Si (001) wafer by photolithography and plasma etching

film pasted on a copper mesh (Cat. No. S110, EM-Japan)

#### *2.2.2 Imaging system*

The diffraction imaging system consists of a diffraction lens (DL) and a projection lens (PL). The diffraction lens focuses the electrons at a back focal plane (BFP), yielding the DPs on the BFP. The projection lens then projects the DPs in the desired magnification onto a viewing screen (scintillator) through a charge-coupled device (CCD) camera, as shown in **Figure 4**. An aperture with a pinhole diameter of 0.5 mm is inserted at the DL center to block scattered electrons and scattered pump laser light. The UED patterns can be observed in two modes: a wide-momentum mode, in which the PL is weak or turned off, and a high-resolution mode, in which the PL magnifies the DPs or images onto the scintillator.

To achieve a high sensitivity to MeV electron detection with a high damage threshold, a Tl-doped CsI columnar crystal scintillator equipped with a fiber optic plate (Hamamatsu Photonics) is used to convert the relativistic-energy DPs or images into optical images. Finally, the optical images are propagated by a thin reflective mirror (at 45°) and detected with an electron-multiplying CCD (EMCCD) with 1024 1024 pixels. The effective detection area of the scintillator is <sup>50</sup> 50 mm<sup>2</sup> , whereas the distance from the specimen to the scintillator is 1.6 m. The sensitivity of the whole detection system is 3 <sup>10</sup><sup>3</sup> counts/electron. The intensity and position of Bragg peaks in the DPs can be monitored and recorded simultaneously using analysis software for studying the structural dynamics.

#### **Figure 4.**

*Schematic of UED imaging with a parallel beam (parallel beam configuration). DL focuses the electrons at a BFP, yielding DPs on BFP. PL then projects the DPs in the desired magnification onto the scintillator. The DPs can be observed in two modes: a wide-momentum mode, in which the PL is weak or turned off, and a high-resolution mode, in which the PL magnifies the DPs or images onto the scintillator.*

## **2.3 Femtosecond laser system**

parallel-beam operation mode with the 0.3-mm-diameter condenser aperture,

maximum (FWHM). The vacuum pressure in the specimen room reaches

the PL magnifies the DPs or images onto the scintillator.

The specimen room is located downstream of the condenser lens. The distance from the photocathode to the specimen is 1.2 m. The sample is manipulated by sixaxis motorized stages. In a time-resolved experiment, the sample can be pumped by a femtosecond laser pulse, as shown in **Figure 2**. A pump laser with wavelengths of 266, 400, and 800 nm can be selected to meet the requirements of the measured materials. The pulse duration of the pump laser pulses is 90 fs in full width at half

<sup>10</sup><sup>10</sup> Torr. When inserting or changing a new sample, the sample is first installed into a separated vacuum chamber (preparation room) for the sample cleaning.

The diffraction imaging system consists of a diffraction lens (DL) and a projection lens (PL). The diffraction lens focuses the electrons at a back focal plane (BFP), yielding the DPs on the BFP. The projection lens then projects the DPs in the desired magnification onto a viewing screen (scintillator) through a charge-coupled device (CCD) camera, as shown in **Figure 4**. An aperture with a pinhole diameter of 0.5 mm is inserted at the DL center to block scattered electrons and scattered pump laser light. The UED patterns can be observed in two modes: a wide-momentum mode, in which the PL is weak or turned off, and a high-resolution mode, in which

To achieve a high sensitivity to MeV electron detection with a high damage threshold, a Tl-doped CsI columnar crystal scintillator equipped with a fiber optic plate (Hamamatsu Photonics) is used to convert the relativistic-energy DPs or images into optical images. Finally, the optical images are propagated by a thin reflective mirror (at 45°) and detected with an electron-multiplying CCD

(EMCCD) with 1024 1024 pixels. The effective detection area of the scintillator is

*Schematic of UED imaging with a parallel beam (parallel beam configuration). DL focuses the electrons at a BFP, yielding DPs on BFP. PL then projects the DPs in the desired magnification onto the scintillator. The DPs can be observed in two modes: a wide-momentum mode, in which the PL is weak or turned off, and a*

*high-resolution mode, in which the PL magnifies the DPs or images onto the scintillator.*

The sensitivity of the whole detection system is 3 <sup>10</sup><sup>3</sup> counts/electron. The intensity and position of Bragg peaks in the DPs can be monitored and recorded simultaneously using analysis software for studying the structural dynamics.

, whereas the distance from the specimen to the scintillator is 1.6 m.

which is discussed in a later section.

*Novel Imaging and Spectroscopy*

*2.2.2 Imaging system*

<sup>50</sup> 50 mm<sup>2</sup>

**Figure 4.**

**16**

A conventional mode-locked Ti:sapphire femtosecond laser (Spectra-Physics) is used to illuminate the photocathode and to excite the sample. The laser consists of a 80-fs Ti:sapphire laser oscillator (Tsunami, central wavelength: 800 nm) and a regenerative amplifier (Spitfire Ace) that includes a pulse stretcher and compressor. The femtosecond laser oscillator is synchronized to an external 79.3-MHz RF signal with a time-to-lock piezoelectric device, as shown in **Figure 2**. The 79.3-MHz RF signal is generated by dividing the accelerating 2856-MHz RF by 1/36. The time jitter between the laser pulse and RF phase is <100 fs. The laser oscillator output is fed to the regenerative amplifier for pulse stretching, amplification, and compression. The regenerative amplifier is driven by a green laser with a repetition rate of 1 kHz (Empower, wavelength: 532 nm, output: 20 W). The pulse energy of the amplifier output is 3 mJ. The pulse duration is 90 fs in FWHM after the pulse compression.

The amplified femtosecond laser beam is divided into two beams. One is converted to the third-harmonics by a wavelength converter (Tripler) composed of two nonlinear crystals (SHG and THG) and a time plate for pulse delay adjustment. The third-harmonic pulses (UV wavelength, 266 nm; pulse duration, 90 fs) with a maximum energy of 5 μJ per pulse are focused by an optical lens and then illuminated onto the copper photocathode to generate femtosecond electron pulses. The residual fundamental femtosecond laser (wavelength: 800 nm) is used directly to excite the sample or is converted to second-harmonics (wavelength: 400 nm) or third-harmonics (wavelength: 266 nm) to excite the sample, based on the sample's requirements. The time delay between the pump laser pulse and the probe electron pulse is changed with an optical delay located on the pump laser beam line for time-resolved experiments, as shown in **Figure 2**. The repetition rate of the pump laser pulses is reduced to 10 Hz with two optical choppers, similar to the repetition rate of the electron pulses.

## **3. UED experiments with relativistic femtosecond electron pulses**

## **3.1 Observations of DPs from crystalline metals, semiconductors, and chemical compounds**

In relativistic UED, we measured the DPs of various crystalline materials (e.g., metals, semiconductors, and chemical compounds). The electron pulses generated from the RF gun were collimated by the condenser aperture with the different pinhole diameters of 1, 0.5, and 0.3 mm before the sample was illuminated. The energy and pulse durations of the electron pulses were 3 MeV and 100 fs, respectively. The following four samples were used:


All DPs of the four samples were observed under the wide-momentum mode.

**Figure 6** shows transmission electron microscopy (TEM) images and UED patterns of a 30-nm-thick polycrystalline aluminum foil (top images) and polycrystalline thallous chloride (bottom images). The DPs were measured with single-shot and 100-pulse integrations. The energy of the electron pulses was 3 MeV. The condenser aperture with a 1-mm diameter pinhole was used to collimate the elec-

indicate that relativistic UED also enabled the electron diffraction imaging of polycrystalline materials and chemical compounds, and the entire DPs were clearly visible with 100 pulses. These suggest that UED with relativistic femtosecond electron pulses enables the study of ultrafast chemical reactions in chemistry and biology. Moreover, single-shot imaging is also possible for polycrystalline materials. This means that the irreversible processes and reactions in polycrystalline materials

**Figures 7** shows the DPs of a (100)-oriented multilayer single-crystalline mica measured with single-shot, 10-, and 100-pulse integrations [29]. The energy of the electron pulses was 3 MeV. The condenser aperture with a 0.5-mm-diameter pinhole was used to collimate the electrons. The number of electrons in the pulse was

. The mica sample was composed of a chemical compound of KAl2(AlSi3) O10(OH)2 with a multilayer structure. The thickness of the monolayer was 10 Å. This is used widely to check the performance of electron diffraction observation in TEM. In general, diffraction imaging of mica is difficult comparing with that of metallic single crystals because of the close diffraction spots. In the relativistic UED measurement as shown in **Figure 7**, both the lowest Bragg and higher-order peaks clearly appear in the observation with 10-pulse integration, and the entire DPs are readily obtained with 100 pulses. Moreover, the possibility of single-shot observa-

*TEM and UED images. Top: a 30-nm thick polycrystalline Al foil. Bottom: polycrystalline thallous chloride. The images of (a) and (b) are measured by a 200-KV TEM. The DPs of (b) and (e) are measured with singleshot, whereas (c) and (f) are measured with 100-pulse integration. The energy of the electron pulses is 3 MeV,*

. The demonstrations

trons. The number of electrons in each pulse was 2.5 <sup>10</sup><sup>7</sup>

*Femtosecond Electron Diffraction Using Relativistic Electron Pulses*

*DOI: http://dx.doi.org/10.5772/intechopen.88511*

and chemical compounds can be observed using relativistic UED.

<sup>6</sup> <sup>10</sup><sup>6</sup>

**Figure 6.**

**19**

*containing 2.5 107 electrons per pulse.*

tion is shown in the measurement.

**Figure 5** shows the DPs of a (001)-orientated single-crystalline Si sample observed both through single-pulse (single-shot) and 10-pulse integrations. A condenser aperture with a 0.3-mm-diameter pinhole was used to collimate the electron beam. The number of electrons in the pulse was <sup>1</sup> <sup>10</sup><sup>6</sup> . In **Figure 5a**, both the lowest Bragg and higher-order peaks clearly appear in the single shot. The excellent quality of the diffraction image is much higher than the pioneering data in the nonrelativistic UED measurement [22]. In **Figure 5b**, entire DPs are clearly visible with 10 pulses integrated. The maximum scattering vector is more than 2 Å<sup>1</sup> .

The intensity profile of Bragg peaks along (440) and (4–40) spots of the single-shot image is shown in **Figure 5c**. The RMS width of the zeroth-order spot (000) was 0.015 Å<sup>1</sup> , indicating an excellent spatial resolution for the MeV diffracted beam. Based on the width of the (000) spot and the measured distance of the diffraction spots from the (000) position, we estimated the RMS illumination convergence angle of the electron beam at the specimen to be α = 26 μrad. By using the 0.3-mm-diameter condenser aperture, we improved both the width and convergence angle from the previous development [20]. The RMS width of both the (220) and (2–20) diffraction spots, which included the effects of the probe beam energy spread, was identical to that of the (000) spot, indicating a small energy spread in the electron pulse generated by the RF gun.

#### **Figure 5.**

*DPs of a (001)-oriented single-crystalline Si measured with (a) single-shot and (b) 10-pulse integrations. (c) The intensity profile along (440) and (4–40) spots of the single-shot image. The solid line represents the raw data, and the broken line is given with 1/10 intensities. The energy of the electron pulses is 3 MeV, containing approximately 1 106 electrons per pulse.*

*Femtosecond Electron Diffraction Using Relativistic Electron Pulses DOI: http://dx.doi.org/10.5772/intechopen.88511*

All DPs of the four samples were observed under the wide-momentum mode. **Figure 5** shows the DPs of a (001)-orientated single-crystalline Si sample observed both through single-pulse (single-shot) and 10-pulse integrations. A condenser aperture with a 0.3-mm-diameter pinhole was used to collimate the electron

lowest Bragg and higher-order peaks clearly appear in the single shot. The excellent quality of the diffraction image is much higher than the pioneering data in the nonrelativistic UED measurement [22]. In **Figure 5b**, entire DPs are clearly visible with 10 pulses integrated. The maximum scattering vector is more than 2 Å<sup>1</sup>

The intensity profile of Bragg peaks along (440) and (4–40) spots of the single-shot image is shown in **Figure 5c**. The RMS width of the zeroth-order spot

diffracted beam. Based on the width of the (000) spot and the measured distance of the diffraction spots from the (000) position, we estimated the RMS illumination convergence angle of the electron beam at the specimen to be α = 26 μrad. By using the 0.3-mm-diameter condenser aperture, we improved both the width and convergence angle from the previous development [20]. The RMS width of both the (220) and (2–20) diffraction spots, which included the effects of the probe beam energy spread, was identical to that of the (000) spot, indicating a small energy

*DPs of a (001)-oriented single-crystalline Si measured with (a) single-shot and (b) 10-pulse integrations. (c) The intensity profile along (440) and (4–40) spots of the single-shot image. The solid line represents the raw data, and the broken line is given with 1/10 intensities. The energy of the electron pulses is 3 MeV, containing*

, indicating an excellent spatial resolution for the MeV

. In **Figure 5a**, both the

.

beam. The number of electrons in the pulse was <sup>1</sup> <sup>10</sup><sup>6</sup>

spread in the electron pulse generated by the RF gun.

(000) was 0.015 Å<sup>1</sup>

*Novel Imaging and Spectroscopy*

**Figure 5.**

**18**

*approximately 1 106 electrons per pulse.*

**Figure 6** shows transmission electron microscopy (TEM) images and UED patterns of a 30-nm-thick polycrystalline aluminum foil (top images) and polycrystalline thallous chloride (bottom images). The DPs were measured with single-shot and 100-pulse integrations. The energy of the electron pulses was 3 MeV. The condenser aperture with a 1-mm diameter pinhole was used to collimate the electrons. The number of electrons in each pulse was 2.5 <sup>10</sup><sup>7</sup> . The demonstrations indicate that relativistic UED also enabled the electron diffraction imaging of polycrystalline materials and chemical compounds, and the entire DPs were clearly visible with 100 pulses. These suggest that UED with relativistic femtosecond electron pulses enables the study of ultrafast chemical reactions in chemistry and biology. Moreover, single-shot imaging is also possible for polycrystalline materials. This means that the irreversible processes and reactions in polycrystalline materials and chemical compounds can be observed using relativistic UED.

**Figures 7** shows the DPs of a (100)-oriented multilayer single-crystalline mica measured with single-shot, 10-, and 100-pulse integrations [29]. The energy of the electron pulses was 3 MeV. The condenser aperture with a 0.5-mm-diameter pinhole was used to collimate the electrons. The number of electrons in the pulse was <sup>6</sup> <sup>10</sup><sup>6</sup> . The mica sample was composed of a chemical compound of KAl2(AlSi3) O10(OH)2 with a multilayer structure. The thickness of the monolayer was 10 Å. This is used widely to check the performance of electron diffraction observation in TEM. In general, diffraction imaging of mica is difficult comparing with that of metallic single crystals because of the close diffraction spots. In the relativistic UED measurement as shown in **Figure 7**, both the lowest Bragg and higher-order peaks clearly appear in the observation with 10-pulse integration, and the entire DPs are readily obtained with 100 pulses. Moreover, the possibility of single-shot observation is shown in the measurement.

#### **Figure 6.**

*TEM and UED images. Top: a 30-nm thick polycrystalline Al foil. Bottom: polycrystalline thallous chloride. The images of (a) and (b) are measured by a 200-KV TEM. The DPs of (b) and (e) are measured with singleshot, whereas (c) and (f) are measured with 100-pulse integration. The energy of the electron pulses is 3 MeV, containing 2.5 107 electrons per pulse.*

#### **Figure 7.**

*DPs of a (100)-oriented multilayer single-crystalline mica measured with (a) single-shot, (b) 10-, and (c) 100-pulse integrations [29]. The energy of the electron pulses is 3 MeV, containing 6* � *106 electrons per pulse.*

In summary, the experiments indicated that (1) the relativistic UED with femtosecond electron pulses can be applied to electron diffraction imaging of a wide range of materials and (2) relativistic UED enables single-shot observation with femtosecond electron pulses. This means that the ultrafast dynamics of irreversible processes in materials can be measured using relativistic UED.

#### **3.2 Time-resolved measurement for study of ultrafast structural dynamics**

A time-resolved measurement technique is used in UED to observe the structural dynamics in a sample. In this case, a femtosecond laser pulse with the desired wavelength based on the sample's requirement is used to excite the sample, whereas the electron pulses are used as a probe to monitor the DPs as a function of the time delay between the pump laser pulse and electron pulse. The time delay in the measurement is adjusted by an optical delay placed on the pump laser beam line, as shown in **Figure 2**. The pulse energy and polarization of the pump laser can be changed to meet the requirements. In the absence of a velocity mismatch between the pump and the probe beams within the sample, the final temporal resolution of UED is expressed as:

$$
\Delta t = \sqrt{\sigma\_b^2 + \sigma\_l^2 + \Delta t\_j^2},
\tag{9}
$$

temporal resolution of the pump-probe measurement was 180 fs. **Figure 8d** gives the plot of the (200) Bragg peak intensity as functions of the pump laser fluence (F) and the time delay between the probe and pump pulses. The intensity was normalized with respect to the value measured for each sample prior to laser excitation. At

*Schematic and results of a time-resolved UED experiment. The UED patterns (a) and (b) are obtained by wide-momentum and high-resolution modes, respectively. (c) The (000)-order peak intensity was obtained by*

*by averaging over four equivalent (200) Bragg-peak spots [25]. Copyright 2013, with permission from the*

To determine the structural changes in the film reflected in the measured dynamics of Bragg peaks, we applied a hybrid method that combines the twotemperature model with classical molecular dynamics (2 T-MD) [26]. This method

at <5 ps and the slower longer-term behavior. At low fluence, *F* = 27 mJ/cm<sup>2</sup> or *F*abs = 3.0 mJ/cm<sup>2</sup> (**Figure 9a** and **c**), the dynamics exhibited premelting of free surfaces and heterogeneous melting by melt front propagation. The melt fronts slowly propagated toward the center, but the film did not melt entirely, and small regions of crystalline gold remained at 1.2 ns. At high fluence, *F* = 41 mJ/cm<sup>2</sup> or *F*abs = 4.5 mJ/cm<sup>2</sup> (**Figure 8b** and **d**), the sample expanded more rapidly, and the premelting was more pronounced. The average temperature reached the melting temperature at 6 ps, and homogenously distributed seeds of low-density molten phase were subsequently created at 6–12 ps. When the sample reached the limit of crystal stability (after 12 ps), these molten seeds grew and coalesced until the

has already been used to model the laser-induced melting of nanofilms and nanorods. By comparing the theoretical predictions from 2 T-MD with the UED measurements, we succeeded in revealing the mechanism of the laser-induced melting. **Figure 9** shows a comparison of experimental and theoretical signals at the pump laser fluences of 27 and 41 mJ/cm<sup>2</sup> and the cross sections of the atomistic simulation of melting in the single-crystal gold film at the absorbed fluences (*F*abs)

intensity was nearly reduced to zero within 15 ps at *<sup>F</sup>* = 45 mJ/cm<sup>2</sup>

*Femtosecond Electron Diffraction Using Relativistic Electron Pulses*

*DOI: http://dx.doi.org/10.5772/intechopen.88511*

strong dependence on the excitation fluence.

, 40% of the intensity was still detectable after 30 ps, whereas the

*. (d) The (200) Bragg peak intensities at F = 1, 27 and 41 mJ/cm<sup>2</sup> were obtained*

. The model reproduced both the fast decay of Bragg intensity

, indicating a

*F* = 27 mJ/cm<sup>2</sup>

*single-shot at F = 27 mJ/cm2*

*American Institute of Physics.*

**Figure 8.**

of 3.0 and 4.5 mJ/cm<sup>2</sup>

sample melted entirely 20 ps.

**21**

where *σ<sup>b</sup>* is the probe electron pulse duration, *σ<sup>l</sup>* is the pump laser pulse duration, and Δ*tj* represents the time jitter between two pulses. The time jitter in the relativistic UED is determined by the synchronization of the laser pulse to the accelerating RF phase. Therefore, we can define Δ*tj* < 100 fs, as discussed in Section 2.3. In the presented time-resolved experiment, both the durations of the pump and probe pulses are �100 fs in RMS, yielding a total temporal resolution of �180 fs in RMS.

The intensities of Bragg peaks measured in the UED experiments reflect both the lattice temperature effect (in terms of the Debye-Waller factor) and details of the structural changes. For example, by monitoring the intensities of Bragg peaks as a function of the time delay between the two pulses, we can investigate the phase transformation in materials (e.g., melting of metal excited by laser light). **Figure 8** presents an example of a time-resolved UED experiment to observe laser-induced melting dynamics in a 10-nm-thick (001)-oriented single-crystal gold film [25, 26]. The sample was excited by a 385-nm femtosecond laser (second-harmonic of a turned 770-nm Ti:sapphire laser). The UED patterns were observed by 3-MeV electron pulses. The electron pulses were collimated to a 200-μm diameter by an aperture in the front of the sample. The diameter of the pump laser at the sample was 800 μm, which was much larger than that of the probe electron beam. The

*Femtosecond Electron Diffraction Using Relativistic Electron Pulses DOI: http://dx.doi.org/10.5772/intechopen.88511*

#### **Figure 8.**

In summary, the experiments indicated that (1) the relativistic UED with femtosecond electron pulses can be applied to electron diffraction imaging of a wide range of materials and (2) relativistic UED enables single-shot observation with femtosecond electron pulses. This means that the ultrafast dynamics of irreversible

*DPs of a (100)-oriented multilayer single-crystalline mica measured with (a) single-shot, (b) 10-, and (c) 100-pulse integrations [29]. The energy of the electron pulses is 3 MeV, containing 6* � *106 electrons per pulse.*

**3.2 Time-resolved measurement for study of ultrafast structural dynamics**

A time-resolved measurement technique is used in UED to observe the structural dynamics in a sample. In this case, a femtosecond laser pulse with the desired wavelength based on the sample's requirement is used to excite the sample, whereas the electron pulses are used as a probe to monitor the DPs as a function of the time delay between the pump laser pulse and electron pulse. The time delay in the measurement is adjusted by an optical delay placed on the pump laser beam line, as shown in **Figure 2**. The pulse energy and polarization of the pump laser can be changed to meet the requirements. In the absence of a velocity mismatch between the pump and the probe beams within the sample, the final temporal resolution of

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

where *σ<sup>b</sup>* is the probe electron pulse duration, *σ<sup>l</sup>* is the pump laser pulse duration, and Δ*tj* represents the time jitter between two pulses. The time jitter in the relativistic UED is determined by the synchronization of the laser pulse to the accelerating RF phase. Therefore, we can define Δ*tj* < 100 fs, as discussed in Section 2.3. In the presented time-resolved experiment, both the durations of the pump and probe pulses are �100 fs in RMS, yielding a total temporal resolution of �180 fs in RMS. The intensities of Bragg peaks measured in the UED experiments reflect both the lattice temperature effect (in terms of the Debye-Waller factor) and details of the structural changes. For example, by monitoring the intensities of Bragg peaks as a function of the time delay between the two pulses, we can investigate the phase transformation in materials (e.g., melting of metal excited by laser light). **Figure 8** presents an example of a time-resolved UED experiment to observe laser-induced melting dynamics in a 10-nm-thick (001)-oriented single-crystal gold film [25, 26]. The sample was excited by a 385-nm femtosecond laser (second-harmonic of a turned 770-nm Ti:sapphire laser). The UED patterns were observed by 3-MeV electron pulses. The electron pulses were collimated to a 200-μm diameter by an aperture in the front of the sample. The diameter of the pump laser at the sample was 800 μm, which was much larger than that of the probe electron beam. The

*<sup>l</sup>* þ Δ*t* 2 *j*

*,* (9)

*σ*2 *<sup>b</sup>* <sup>þ</sup> *<sup>σ</sup>*<sup>2</sup>

q

processes in materials can be measured using relativistic UED.

Δ*t* ¼

UED is expressed as:

**20**

**Figure 7.**

*Novel Imaging and Spectroscopy*

*Schematic and results of a time-resolved UED experiment. The UED patterns (a) and (b) are obtained by wide-momentum and high-resolution modes, respectively. (c) The (000)-order peak intensity was obtained by single-shot at F = 27 mJ/cm2 . (d) The (200) Bragg peak intensities at F = 1, 27 and 41 mJ/cm<sup>2</sup> were obtained by averaging over four equivalent (200) Bragg-peak spots [25]. Copyright 2013, with permission from the American Institute of Physics.*

temporal resolution of the pump-probe measurement was 180 fs. **Figure 8d** gives the plot of the (200) Bragg peak intensity as functions of the pump laser fluence (F) and the time delay between the probe and pump pulses. The intensity was normalized with respect to the value measured for each sample prior to laser excitation. At *F* = 27 mJ/cm<sup>2</sup> , 40% of the intensity was still detectable after 30 ps, whereas the intensity was nearly reduced to zero within 15 ps at *<sup>F</sup>* = 45 mJ/cm<sup>2</sup> , indicating a strong dependence on the excitation fluence.

To determine the structural changes in the film reflected in the measured dynamics of Bragg peaks, we applied a hybrid method that combines the twotemperature model with classical molecular dynamics (2 T-MD) [26]. This method has already been used to model the laser-induced melting of nanofilms and nanorods. By comparing the theoretical predictions from 2 T-MD with the UED measurements, we succeeded in revealing the mechanism of the laser-induced melting. **Figure 9** shows a comparison of experimental and theoretical signals at the pump laser fluences of 27 and 41 mJ/cm<sup>2</sup> and the cross sections of the atomistic simulation of melting in the single-crystal gold film at the absorbed fluences (*F*abs) of 3.0 and 4.5 mJ/cm<sup>2</sup> . The model reproduced both the fast decay of Bragg intensity at <5 ps and the slower longer-term behavior. At low fluence, *F* = 27 mJ/cm<sup>2</sup> or *F*abs = 3.0 mJ/cm<sup>2</sup> (**Figure 9a** and **c**), the dynamics exhibited premelting of free surfaces and heterogeneous melting by melt front propagation. The melt fronts slowly propagated toward the center, but the film did not melt entirely, and small regions of crystalline gold remained at 1.2 ns. At high fluence, *F* = 41 mJ/cm<sup>2</sup> or *F*abs = 4.5 mJ/cm<sup>2</sup> (**Figure 8b** and **d**), the sample expanded more rapidly, and the premelting was more pronounced. The average temperature reached the melting temperature at 6 ps, and homogenously distributed seeds of low-density molten phase were subsequently created at 6–12 ps. When the sample reached the limit of crystal stability (after 12 ps), these molten seeds grew and coalesced until the sample melted entirely 20 ps.

electron beam with a small condenser aperture of 0.1 mm or less. In this case, the spatial coherence length can be improved to *Lc* > 10 nm, which is an ideal value for electron diffraction imaging with the highest spatial resolution. In addition, as discussed in Section 3.2, the atomic-level analytical method of 2 T-MD is very useful to explain the atomic dynamics and underlying mechanisms. Through combination with the 2 T-MD method, relativistic UED can be a powerful tool for femtosecond

The author acknowledges K. Kan, Y. Yoshida, H. Yasuda, and K. Tanimura of Osaka University and Y. Naruse of the Shiga University of Medical Science for their valuable suggestions and discussions. In addition, the author thanks J. Urakawa, T. Takatomi and N. Terunuma of the High Energy Accelerator Research Organiza-

This work was supported by a Basic Research (A) (No. 22246127, No. 26246026, and No. 17H01060) of Grant-in-Aid for Scientific Research from MEXT, Japan.

The Institute of Scientific and Industrial Research, Osaka University, Osaka, Japan

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

\*Address all correspondence to: yang@sanken.osaka-u.ac.jp

provided the original work is properly cited.

tion (KEK) for the design and fabrication of the high-quality RF gun.

imaging in materials science, chemistry, and biology.

*Femtosecond Electron Diffraction Using Relativistic Electron Pulses*

*DOI: http://dx.doi.org/10.5772/intechopen.88511*

**Acknowledgements**

**Author details**

Jinfeng Yang

**23**

#### **Figure 9.**

*Cross-sections (top) of the atomistic simulation of meltings in single crystal gold films at (a) Fabs = 3.0 and (b) 4.5 mJ/cm<sup>2</sup> , and a comparison (bottom) of experimental and theoretical signals as a function of time delay at (c) F = 27 and (d) 41 mJ/cm2 . The dots denote the experimental data, whereas the lines represent the theoretical results. In (a) and (b), all atoms are color-coded according to the measure of a degree of crystallinity given by the (nearest-neighbor averaged) centro-symmetry parameter Φ. Blue atoms (Φ < 0.45) have a local crystalline structure, and red atoms (Φ* ≥ *0.45) have highly disordered surroundings [25]. Copyright 2013, with permission from the American Institute of Physics.*

Recently, other time-resolved observations of ultrafast structural dynamics in semiconductors, organic crystals, and two-dimensional materials have been proposed in relativistic UED. The results showed that relativistic UED has great potential and excellent performance for the study of ultrafast photo-induced phase transitions.

## **4. Conclusion**

In this chapter, we introduced femtosecond diffraction imaging with relativistic electron pulses. We also discussed relativistic femtosecond electron pulse generation using an RF-acceleration-based photoemission gun, a relativistic UED apparatus, and demonstrations of UED measurements with the relativistic femtosecond electron pulses. The electron pulses generated by the RF gun exhibited excellent characteristics, including a low emittance of ≤0.1 mm-mrad, a short pulse duration of 100 fs, and a high number of electrons of ≥10<sup>6</sup> in the pulse at an energy of 3 MeV. The peak brightness was reached at 10<sup>22</sup> electrons/m<sup>2</sup> sr. These pulses facilitated (1) the acquisition of high-quality DPs with a spatial resolution of 0.015 Å<sup>1</sup> and (2) a time-resolved experiment (pump-probe measurement) with an excellent temporal resolution of 180 fs. The most important result was the single-shot observation with relativistic femtosecond electron pulses in a wide range of materials. The findings suggest that relativistic UED is very promising for the study of ultrafast dynamics of irreversible processes and chemical reactions (not discussed in this chapter) in the femtosecond time region.

In addition, the UED system is also very stable because it combines the current state-of-the-art femtosecond mode-locked laser and RF-acceleration and timesynchronization technologies. In our study, the fluctuation of the Bragg peak intensity was <5% in the long term (i.e., 24 hours or more). The relativistic UED apparatus is also very compact.

In the future, an ultralow emittance of 10 nm-mrad can be expected by focusing the μm-diameter laser spots on the photocathode and collimating the *Femtosecond Electron Diffraction Using Relativistic Electron Pulses DOI: http://dx.doi.org/10.5772/intechopen.88511*

electron beam with a small condenser aperture of 0.1 mm or less. In this case, the spatial coherence length can be improved to *Lc* > 10 nm, which is an ideal value for electron diffraction imaging with the highest spatial resolution. In addition, as discussed in Section 3.2, the atomic-level analytical method of 2 T-MD is very useful to explain the atomic dynamics and underlying mechanisms. Through combination with the 2 T-MD method, relativistic UED can be a powerful tool for femtosecond imaging in materials science, chemistry, and biology.

## **Acknowledgements**

Recently, other time-resolved observations of ultrafast structural dynamics in semiconductors, organic crystals, and two-dimensional materials have been proposed in relativistic UED. The results showed that relativistic UED has great potential and excellent performance for the study of ultrafast photo-induced phase

*Cross-sections (top) of the atomistic simulation of meltings in single crystal gold films at (a) Fabs = 3.0 and*

*theoretical results. In (a) and (b), all atoms are color-coded according to the measure of a degree of crystallinity given by the (nearest-neighbor averaged) centro-symmetry parameter Φ. Blue atoms (Φ < 0.45) have a local crystalline structure, and red atoms (Φ* ≥ *0.45) have highly disordered surroundings [25]. Copyright 2013,*

*, and a comparison (bottom) of experimental and theoretical signals as a function of time delay*

*. The dots denote the experimental data, whereas the lines represent the*

In this chapter, we introduced femtosecond diffraction imaging with relativistic electron pulses. We also discussed relativistic femtosecond electron pulse generation using an RF-acceleration-based photoemission gun, a relativistic UED apparatus, and demonstrations of UED measurements with the relativistic femtosecond electron pulses. The electron pulses generated by the RF gun exhibited excellent characteristics, including a low emittance of ≤0.1 mm-mrad, a short pulse duration of 100 fs, and a high number of electrons of ≥10<sup>6</sup> in the pulse at an energy of 3 MeV. The peak brightness was reached at 10<sup>22</sup> electrons/m<sup>2</sup> sr. These pulses facilitated (1) the acquisition of high-quality DPs with a spatial resolution of 0.015 Å<sup>1</sup> and (2) a time-resolved experiment (pump-probe measurement) with an excellent temporal resolution of 180 fs. The most important result was the single-shot observation with relativistic femtosecond electron pulses in a wide range of materials. The findings suggest that relativistic UED is very promising for the study of ultrafast dynamics of irreversible processes and chemical reactions (not discussed in this chapter) in the

In addition, the UED system is also very stable because it combines the current state-of-the-art femtosecond mode-locked laser and RF-acceleration and timesynchronization technologies. In our study, the fluctuation of the Bragg peak intensity was <5% in the long term (i.e., 24 hours or more). The relativistic UED

In the future, an ultralow emittance of 10 nm-mrad can be expected by focusing the μm-diameter laser spots on the photocathode and collimating the

transitions.

**Figure 9.**

*(b) 4.5 mJ/cm<sup>2</sup>*

*at (c) F = 27 and (d) 41 mJ/cm2*

*Novel Imaging and Spectroscopy*

*with permission from the American Institute of Physics.*

**4. Conclusion**

femtosecond time region.

**22**

apparatus is also very compact.

The author acknowledges K. Kan, Y. Yoshida, H. Yasuda, and K. Tanimura of Osaka University and Y. Naruse of the Shiga University of Medical Science for their valuable suggestions and discussions. In addition, the author thanks J. Urakawa, T. Takatomi and N. Terunuma of the High Energy Accelerator Research Organization (KEK) for the design and fabrication of the high-quality RF gun.

This work was supported by a Basic Research (A) (No. 22246127, No. 26246026, and No. 17H01060) of Grant-in-Aid for Scientific Research from MEXT, Japan.

## **Author details**

Jinfeng Yang The Institute of Scientific and Industrial Research, Osaka University, Osaka, Japan

\*Address all correspondence to: yang@sanken.osaka-u.ac.jp

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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10.1103/PhysRevLett.52.2364

[9] Ihee H et al. Direct imaging of transient molecular structures with ultrafast diffraction. Science. 2001;**291**:

458-462. DOI: 10.1126/science.

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Physics. 2004;**299**:285-305. DOI: 10.1016/j.chemphys.2003.11.040

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0034-4885/74/9/096101

femtosecond electron packets. Chemical

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pnas.1010165107

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[2] Siwick BJ, Dwyer JR, Jordan RE, Dwayne Miller RJ. An atomic-level view of melting using femtosecond electron

[3] King WE et al. Ultrafast electron microscopy in materials science, biology, and chemistry. Journal of Applied Physics. 2005;**97**:111101. DOI:

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[5] Ewbank JD et al. Instrumentation for gas electron diffraction employing a pulsed electron beam synchronous with

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Picosecond electron diffraction. Applied Physics Letters. 1982;**41**:44-45. DOI:

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photoexcitation. The Review of Scientific Instruments. 1992;**63**: 3352-3358. DOI: 10.1063/1.1142552

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10.1063/1.93316

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m.57.032905.104748

10.1063/1.1927699

f29848001145

1090052

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[21] Zhu P, Zhu Y, Hidaka Y, Wu L, Cao J, Berger H, et al. Femtosecond time-resolved MeV electron diffraction. New Journal of Physics. 2015;**17**: 063004. DOI: 10.1088/1367-2630/17/6/ 063004

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[34] Terunuma N, Murata A, Fukuda M, Hirano K, Kamiya Y, Kii T, et al. Improvement of an S-band RF gun with a Cs2Te photocathode for the KEK-ATF. Nuclear Instruments and Methods in Physics Research Section A. 2010;**613**: 1-8. DOI: 10.1016/j.nima.2009.10.151

[35] Yang J, Sakai F, Yanagida T, Yorozu M, Okada Y, Takasago K, et al. Low-emittance electron-beam generation with laser pulse shaping in photocathode radio-frequency gun. Journal of Applied Physics. 2002;**92**: 1608-1612. DOI: 10.1063/1.1487457

[36] Yang J, Kan K, Kondoh T, Murooka Y, Naruse N, Yoshida Y, et al. An ultrashort-bunch electron RF gun. Journal of the Vacuum Society of Japan. 2012;**55**:42-49. DOI: 10.3131/jvsj2.55.42

[37] Van Oudheusden T, de Jong EF, van der Geer SB. Op't root WPEM, Luiten OJ. Electron source concept for single-shot sub-100 fs electron diffraction in the 100 keV range. Journal of Applied Physics. 2007;**102**:093501. DOI: 10.1063/1.2801027

[38] Gahlmann A, Park AT, Zewail AH. Ultrashort electron pulses for diffraction, crystallography and microscopy: Theoretical and experimental resolutions. Physical Chemistry Chemical Physics. 2008;**10**: 2894-2909. DOI: 10.1039/b802136h

**Chapter 3**

**Abstract**

radiation chemistry

**1. Introduction**

**27**

pulse radiolysis development, are presented.

**1.1 Primary processes in radiation chemistry**

in nonpolar materials like alkenes [4–10].

in the radiation chemistry [11, 12] can be summarized as:

*e*

Femtosecond Pulse Radiolysis

*Jinfeng Yang, Koichi Kan, Masao Gohdo and Yoichi Yoshida*

Ultrafast pulse radiolysis with a short-pulsed electron beam and a short-pulsed analyzing light is a powerful time-resolved spectroscopic technique to study the kinetics and reactions of short-lived intermediate species or precursors in radiation chemistry and biology. In this chapter, first, we give an overview of historical developments of ultrafast pulse radiolysis. Then, we describe a femtosecond pulse radiolysis instrument, including the generation of femtosecond electron pulses by a photocathode radio frequency (rf) gun-based linear electron accelerator, the synchronization of femtosecond analyzing laser with the electron pulses, the transient absorption measurement with double-pulse technique, and the observations of the formation processes and ultrafast reactions of hydrated electrons in water. Finally, two innovative techniques, which enable to improve the time resolution in next

**Keywords:** pulse radiolysis, femtosecond electron pulse, femtosecond laser, short-lived intermediate species, precursors, hydrated electron, ultrafast reaction,

Primary processes or ultrafast reactions in radiation chemistry and biology are occurred in the ionization of molecules with radiation beams, for example, gammaor X-rays, electrons, ions, and other high-energy particles. The ionization of molecules produces a positive radical ion and an electron with an initial kinetic energy.

surrounded by solvent molecules to form solvated electron state in polar materials like water [1–3], or reacted with the positive radical ion by geminate recombination

� þ nH2O ! *e*

where Eq. (1) represents the ionization and electronic excitation of water molecules on a attosecond timescale (�10�<sup>16</sup> s), Eq. (2) is the ion-molecular

For example, in water, the primary processes at the current state of knowledge

H2O <sup>⟿</sup> H2O<sup>∙</sup><sup>þ</sup> <sup>þ</sup> <sup>e</sup>�, H2O <sup>⟿</sup> H2O<sup>∗</sup> (1) H2O<sup>∙</sup><sup>þ</sup> <sup>þ</sup> H2O ! OH<sup>∙</sup> <sup>þ</sup> H3O<sup>þ</sup> (2)

H2O<sup>∗</sup> ! OH<sup>∙</sup> <sup>þ</sup> <sup>H</sup><sup>∙</sup> (3)

�*hyd* (4)

The electron will be thermalized by interactions with molecules, and then

## **Chapter 3**

[31] Yang J, Yoshida Y. Relativistic ultrafast electron microscopy: Single-

femtosecond electron pulses. Advances in Condensed Matter Physics. 2019; **2019**:9739241. DOI: 10.1155/2019/

[38] Gahlmann A, Park AT, Zewail AH.

Ultrashort electron pulses for diffraction, crystallography and microscopy: Theoretical and experimental resolutions. Physical Chemistry Chemical Physics. 2008;**10**: 2894-2909. DOI: 10.1039/b802136h

[32] Arita M, Sakaguchi N. Electron microscopy: Novel microscopy trends. In: Yang J, editor. Ultrafast Electron Microscopy with Relativistic Femtosecond Electron Pulses. InTechOpen; 2019. DOI: 10.1093/

[33] Kan K, Yang J, Kondoh T, Yoshida Y. Development of

femtosecond photocathode RF gun. Nuclear Instruments and Methods in Physics Research Section A. 2011;**659**: 44-48. DOI: 10.1016/j.nima.2011.08.016

[34] Terunuma N, Murata A, Fukuda M,

Improvement of an S-band RF gun with a Cs2Te photocathode for the KEK-ATF. Nuclear Instruments and Methods in Physics Research Section A. 2010;**613**: 1-8. DOI: 10.1016/j.nima.2009.10.151

Hirano K, Kamiya Y, Kii T, et al.

[35] Yang J, Sakai F, Yanagida T, Yorozu M, Okada Y, Takasago K, et al.

Low-emittance electron-beam

[36] Yang J, Kan K, Kondoh T,

der Geer SB. Op't root WPEM,

single-shot sub-100 fs electron

DOI: 10.1063/1.2801027

**26**

generation with laser pulse shaping in photocathode radio-frequency gun. Journal of Applied Physics. 2002;**92**: 1608-1612. DOI: 10.1063/1.1487457

Murooka Y, Naruse N, Yoshida Y, et al. An ultrashort-bunch electron RF gun. Journal of the Vacuum Society of Japan. 2012;**55**:42-49. DOI: 10.3131/jvsj2.55.42

[37] Van Oudheusden T, de Jong EF, van

Luiten OJ. Electron source concept for

diffraction in the 100 keV range. Journal of Applied Physics. 2007;**102**:093501.

shot diffraction imaging with

*Novel Imaging and Spectroscopy*

9739241

jmicro/dfy032

## Femtosecond Pulse Radiolysis

*Jinfeng Yang, Koichi Kan, Masao Gohdo and Yoichi Yoshida*

## **Abstract**

Ultrafast pulse radiolysis with a short-pulsed electron beam and a short-pulsed analyzing light is a powerful time-resolved spectroscopic technique to study the kinetics and reactions of short-lived intermediate species or precursors in radiation chemistry and biology. In this chapter, first, we give an overview of historical developments of ultrafast pulse radiolysis. Then, we describe a femtosecond pulse radiolysis instrument, including the generation of femtosecond electron pulses by a photocathode radio frequency (rf) gun-based linear electron accelerator, the synchronization of femtosecond analyzing laser with the electron pulses, the transient absorption measurement with double-pulse technique, and the observations of the formation processes and ultrafast reactions of hydrated electrons in water. Finally, two innovative techniques, which enable to improve the time resolution in next pulse radiolysis development, are presented.

**Keywords:** pulse radiolysis, femtosecond electron pulse, femtosecond laser, short-lived intermediate species, precursors, hydrated electron, ultrafast reaction, radiation chemistry

## **1. Introduction**

#### **1.1 Primary processes in radiation chemistry**

Primary processes or ultrafast reactions in radiation chemistry and biology are occurred in the ionization of molecules with radiation beams, for example, gammaor X-rays, electrons, ions, and other high-energy particles. The ionization of molecules produces a positive radical ion and an electron with an initial kinetic energy. The electron will be thermalized by interactions with molecules, and then surrounded by solvent molecules to form solvated electron state in polar materials like water [1–3], or reacted with the positive radical ion by geminate recombination in nonpolar materials like alkenes [4–10].

For example, in water, the primary processes at the current state of knowledge in the radiation chemistry [11, 12] can be summarized as:

$$\text{H}\_2\text{O} \leadsto \text{H}\_2\text{O}^{\ast+} + \text{e}^-, \text{H}\_2\text{O} \leadsto \text{H}\_2\text{O}^\* \tag{1}$$

$$\text{H}\_2\text{O}^{\bullet+} + \text{H}\_2\text{O} \rightarrow \text{OH}^\bullet + \text{H}\_3\text{O}^+\tag{2}$$

$$\mathrm{H}\_{2}\mathrm{O}^{\*} \rightarrow \mathrm{OH}^{\*} + \mathrm{H}^{\*} \tag{3}$$

$$e^- + \text{nH}\_2\text{O} \rightarrow e^-\_{hyd} \tag{4}$$

where Eq. (1) represents the ionization and electronic excitation of water molecules on a attosecond timescale (�10�<sup>16</sup> s), Eq. (2) is the ion-molecular

reaction of positive radical ion H2O<sup>∙</sup><sup>+</sup> in <sup>10</sup><sup>14</sup> s, Eq. (3) is the dissociation of electronic excited state H2O\* in <sup>10</sup><sup>13</sup> s, and Eq. (4) is the formation process of solvated electron (called also hydrated electron in water, denoted by *e hyd*) in <sup>10</sup><sup>13</sup> s.

Another linac was used to produce picosecond single-pulse Cherenkov light. In 1990s, a picosecond pulse radiolysis system using the laser diode instead of the Cherenkov light was developed [18]. The system enabled to measure the transient absorption from the visible to near-infrared region. In the late 1990s, a femtosecond laser was used for the analyzing light to cover the wide wavelength range from ultraviolet to infrared [19]. However, in spite of many innovative techniques that have been developed, the time resolution had remained at 20 ps since the 1970s. A remarkable progress in ultrafast pulse radiolysis has been made since 2000. A sub-picosecond pulse radiolysis was developed at Osaka University using a subpicosecond single-pulse electron beam and a femtosecond laser light [20]. The sub-

picosecond single-pulse electron beam was generated by a 28-MeV L-band (1300 MHz) electron linac and a magnetic pulse compressor. A synchronized Ti: sapphire laser with pulse width of 60 fs was used as analyzing light source. Moreover, a time jitter compensation technique was developed by directly detecting the time interval between the electron pulse and the analyzing laser light with a femto-

second streak camera. Finally, the time resolution of 2 ps was achieved.

ultrafast pulse radiolysis facilities using the photocathode rf guns.

University of Tokyo 1.6-cell rf gun, booster linac,

University of Paris-Sud 1.5-cell rf gun, booster linac,

Waseda University 1.6-cell rf gun, 4.8-ps white

Osaka University 1.6-cell rf gun, booster linac,

*The ultrafast pulse radiolysis facilities using photocathode rf guns.*

Brookhaven National Laboratory

*Femtosecond Pulse Radiolysis*

*DOI: http://dx.doi.org/10.5772/intechopen.91691*

Sumitomo Heavy Industries

**Table 1.**

**29**

In the 2000s, a new type of ultrafast pulse radiolysis using an advanced accelerator technology of laser-triggered photocathode radio frequency (rf) electron gun was developed at the Brookhaven National Laboratory, at the University of Tokyo, at Sumitomo Heavy Industries, at the University of Paris-Sud, at Waseda University, and at Osaka University. The photocathode rf gun consists of two to four rf cavities driven with high rf power to accelerate the electrons emitted from the photocathode to an energy region of 49 MeV. The rf gun enables to generate a picosecond single-pulsed electron beam with electron charge of nano-coulombs using picosecond laser excitation on the photocathode. The new type of pulse radiolysis exhibits two crucial advantages: (1) the instrument is very compact without the need of the sub-harmonic bunching system and (2) the electron pulses produced by the rf gun are time-synchronized with laser pulses, resulting in a low time jitter between the electron pulse and the analyzing laser light. **Table 1** gives the

In 2010, the first femtosecond pulse radiolysis with time resolution of 240 fs was successfully developed at Osaka University using a femtosecond single-pulsed electron beam generated by a 32-MeV photocathode rf gun linac and a magnetic pulse compressor [26–29]. The femtosecond pulse radiolysis was used successfully to observe the femtosecond solvation dynamics of hydrated electrons in water.

**Facility Instrument Electron pulse characteristics Refs.**

100-fs laser

120-fs laser

light

107-fs laser

3.5-cell rf gun, 100-fs laser ≥7 ps, 68 nC, 8.7 MeV [21]

1.6-cell rf gun, 15-ps laser 20 ps, 1 nC, 1.75 MeV [23]

7 ps, 0.65 nC, 18 MeV, time jitter: 2.1 ps

<5 ps, <2 nC, 10 ps, 5 nC, 49 MeV

98400 fs, 0.11 nC, 32 MeV, time jitter: 61 fs

6 ps, 1 nC, 4.5 MeV [25]

[22]

[24]

[26–29]

To study such ultrafast kinetics and reactions, two techniques have been developed: photolysis and pulse radiolysis. The photolysis is mainly used in photochemistry, while pulse radiolysis is widely used in radiation chemistry. In photolysis, short-lived intermediate species or precursors are produced by photoionization with high-energy photons or multiphoton excitation. The species and their reactions are measured by time-resolved absorption spectroscopy with analyzing laser light. The first observation of femtosecond solvation dynamics of hydrated electrons in water was achieved in 1987 by Migus et al. by photolysis with a femtosecond laser [1]. In pulse radiolysis, short-pulsed radiation, that is, pulsed X-rays, electrons, or ions, ionizes molecules. The short-lived intermediate species and primary processes are observed by transient absorption spectroscopy with an analyzing light. The first pulse radiolysis was developed in 1962 by Hart and Boag [13] using 1.8-MeV and 3-μs-long electron pulses generated by an electron accelerator. They succeeded firstly in the direct spectroscopic observation of the solvated electrons in aqueous solutions. The experimental results indicated that pulse radiolysis is a very promising technique to determine short-lived intermediate species or precursors and observe their kinetics or reactions.

## **1.2 Historical developments of ultrafast pulse radiolysis**

Progress of particle accelerator technology pays a significant role in the ultrafast pulse radiolysis development. The first picosecond pulse radiolysis was developed at the University of Toronto in the late 1960s [14] using the fine structure of 30-ps electron pulse train with the duration of 30 ns generated by S-band (2856 MHz) linear electron accelerator (or linac). Cherenkov light emitted by the picosecond electron pulses was used as analyzing light to detect the transient absorption, as shown in **Figure 1**. However, in this pulse radiolysis, the sample was irradiated every 350 ps (one period of S-band microwave) by the electron pulse train. Reactions or kinetics with time constant of >350 ps overlapped in the measurement. To circumvent this problem, several picosecond pulse radiolysis facilities using picosecond single-pulse electron accelerators with a sub-harmonic bunching technique were constructed in 1970s and 1980s. Tabata et al. at the University of Tokyo developed a 20-ps pulse radiolysis with two parallel linacs [15–17]. One linac was used to generate a picosecond single-pulse electron beam to irradiate the sample.

#### **Figure 1.**

*First picosecond pulse radiolysis with time resolution of 23 ps developed at the University of Toronto in the late 1960s [14].*

## *Femtosecond Pulse Radiolysis DOI: http://dx.doi.org/10.5772/intechopen.91691*

reaction of positive radical ion H2O<sup>∙</sup><sup>+</sup> in <sup>10</sup><sup>14</sup> s, Eq. (3) is the dissociation of electronic excited state H2O\* in <sup>10</sup><sup>13</sup> s, and Eq. (4) is the formation process of

To study such ultrafast kinetics and reactions, two techniques have been developed: photolysis and pulse radiolysis. The photolysis is mainly used in photochemistry, while pulse radiolysis is widely used in radiation chemistry. In photolysis, short-lived intermediate species or precursors are produced by photoionization with high-energy photons or multiphoton excitation. The species and their reactions are measured by time-resolved absorption spectroscopy with analyzing laser light. The first observation of femtosecond solvation dynamics of hydrated electrons in water was achieved in 1987 by Migus et al. by photolysis with a femtosecond laser [1]. In pulse radiolysis, short-pulsed radiation, that is, pulsed X-rays, electrons, or ions, ionizes molecules. The short-lived intermediate species and primary processes are observed by transient absorption spectroscopy with an analyzing light. The first pulse radiolysis was developed in 1962 by Hart and Boag [13] using 1.8-MeV and 3-μs-long electron pulses generated by an electron accelerator. They succeeded firstly in the direct spectroscopic observation of the solvated electrons in aqueous solutions. The experimental results indicated that pulse radiolysis is a very promising technique to determine short-lived intermediate

Progress of particle accelerator technology pays a significant role in the ultrafast pulse radiolysis development. The first picosecond pulse radiolysis was developed at the University of Toronto in the late 1960s [14] using the fine structure of 30-ps electron pulse train with the duration of 30 ns generated by S-band (2856 MHz) linear electron accelerator (or linac). Cherenkov light emitted by the picosecond electron pulses was used as analyzing light to detect the transient absorption, as shown in **Figure 1**. However, in this pulse radiolysis, the sample was irradiated every 350 ps (one period of S-band microwave) by the electron pulse train.

Reactions or kinetics with time constant of >350 ps overlapped in the measurement. To circumvent this problem, several picosecond pulse radiolysis facilities using picosecond single-pulse electron accelerators with a sub-harmonic bunching technique were constructed in 1970s and 1980s. Tabata et al. at the University of Tokyo developed a 20-ps pulse radiolysis with two parallel linacs [15–17]. One linac was used to generate a picosecond single-pulse electron beam to irradiate the sample.

*First picosecond pulse radiolysis with time resolution of 23 ps developed at the University of Toronto in the late*

 *hyd*) in

solvated electron (called also hydrated electron in water, denoted by *e*

species or precursors and observe their kinetics or reactions.

**1.2 Historical developments of ultrafast pulse radiolysis**

<sup>10</sup><sup>13</sup> s.

*Novel Imaging and Spectroscopy*

**Figure 1.**

*1960s [14].*

**28**

Another linac was used to produce picosecond single-pulse Cherenkov light. In 1990s, a picosecond pulse radiolysis system using the laser diode instead of the Cherenkov light was developed [18]. The system enabled to measure the transient absorption from the visible to near-infrared region. In the late 1990s, a femtosecond laser was used for the analyzing light to cover the wide wavelength range from ultraviolet to infrared [19]. However, in spite of many innovative techniques that have been developed, the time resolution had remained at 20 ps since the 1970s.

A remarkable progress in ultrafast pulse radiolysis has been made since 2000. A sub-picosecond pulse radiolysis was developed at Osaka University using a subpicosecond single-pulse electron beam and a femtosecond laser light [20]. The subpicosecond single-pulse electron beam was generated by a 28-MeV L-band (1300 MHz) electron linac and a magnetic pulse compressor. A synchronized Ti: sapphire laser with pulse width of 60 fs was used as analyzing light source. Moreover, a time jitter compensation technique was developed by directly detecting the time interval between the electron pulse and the analyzing laser light with a femtosecond streak camera. Finally, the time resolution of 2 ps was achieved.

In the 2000s, a new type of ultrafast pulse radiolysis using an advanced accelerator technology of laser-triggered photocathode radio frequency (rf) electron gun was developed at the Brookhaven National Laboratory, at the University of Tokyo, at Sumitomo Heavy Industries, at the University of Paris-Sud, at Waseda University, and at Osaka University. The photocathode rf gun consists of two to four rf cavities driven with high rf power to accelerate the electrons emitted from the photocathode to an energy region of 49 MeV. The rf gun enables to generate a picosecond single-pulsed electron beam with electron charge of nano-coulombs using picosecond laser excitation on the photocathode. The new type of pulse radiolysis exhibits two crucial advantages: (1) the instrument is very compact without the need of the sub-harmonic bunching system and (2) the electron pulses produced by the rf gun are time-synchronized with laser pulses, resulting in a low time jitter between the electron pulse and the analyzing laser light. **Table 1** gives the ultrafast pulse radiolysis facilities using the photocathode rf guns.

In 2010, the first femtosecond pulse radiolysis with time resolution of 240 fs was successfully developed at Osaka University using a femtosecond single-pulsed electron beam generated by a 32-MeV photocathode rf gun linac and a magnetic pulse compressor [26–29]. The femtosecond pulse radiolysis was used successfully to observe the femtosecond solvation dynamics of hydrated electrons in water.


#### **Table 1.**

*The ultrafast pulse radiolysis facilities using photocathode rf guns.*

It paves the way to observe the short-lived intermediate species and primary processes in radiation chemistry on femtosecond timescale.

Recently, a technique of laser wakefield electron acceleration instead of the traditional accelerators was applied in ultrafast pulse radiolysis at the Argonne National Laboratory [30]. However, the time resolution is still limited to picosecond because of the electron pulse duration, the time jitter, and the effect due to group velocity mismatch (GVM) between the electron beam and the light in the sample.

The progress of the ultrafast laser technology allows us to use a femtosecond laser as an analyzing light source in pulse radiolysis. However, in order to achieve a femto-

3.detecting the transient absorption with a good signal-to-noise (S/N) ratio in a

In this section, we focus on the generation of femtosecond electron pulses with a

Femtosecond pulse radiolysis at Osaka University was constructed with a 1.6-cell S-band (2.856 GHz) photocathode rf gun, a booster linac, a magnetic pulse compressor, and an analyzing femtosecond Ti:sapphire laser, as shown in **Figure 3**. The photocathode rf gun used in femtosecond pulse radiolysis contains two cavities: a half cell and a full cell. The length of the half-cell cavity was equal to 0.6

*Schematic of femtosecond pulse radiolysis apparatus at Osaka University, containing a photocathode rf gun, a booster linac, a magnetic pulse compressor, and an analyzing femtosecond Ti:sapphire laser. The rf gun was driven by a picosecond Nd:YLF laser. A time resolution of 240 fs due to the electron beam and the analyzing*

laser photocathode electron gun accelerator, ultrafast pulse radiolysis with the femtosecond-pulsed electron beam and a femtosecond laser, and a double-pulse detection technique to observe the transient absorption in a thin sample cell.

second time resolution, three significant techniques are indispensable:

2. synchronizing the electron pulse with the analyzing light pulse in

1.generating a femtosecond-pulsed electron beam,

thin sample cell to circumvent the effect of GVM.

femtosecond, and

*Femtosecond Pulse Radiolysis*

*DOI: http://dx.doi.org/10.5772/intechopen.91691*

**2.1 1.6-cell photocathode rf gun**

**Figure 3.**

**31**

*laser has been achieved [26–29].*

In order to reduce furthermore the time jitter between the electron pulse and the analyzing light pulse, a technique of a "double-decker electron beam accelerator" was proposed in pulse radiolysis at Osaka University [31, 32]. The entirely synchronized double-decker electron beams with a time interval of 1.4 ns were generated in the photocathode rf gun with 2-ps laser beams, then accelerated to 32 MeV with a booster linac, and finally compressed into femtosecond with a magnetic compressor. One of the double-decker electron beams was used to irradiate the sample, while another beam was converted to the analyzing light, for example, Cherenkov light or terahertz (THz) light, resulting in the generation of entirely synchronized femtosecond-pulsed electron beam and femtosecond analyzing light. Moreover, in order to circumvent the effect of GVM, an innovative technique of "equivalent velocity spectroscopy (EVS)" was developed for pulse radiolysis at Osaka University [33]. The method of ultrafast pulse radiolysis was reviewed in a book entitled "Recent trends in radiation chemistry" edited by Wishart and Rao in 2010 [34].

In this chapter, we describe the details of femtosecond pulse radiolysis, including the generation of femtosecond electron pulses, the synchronization of femtosecond analyzing laser with the electron pulses, double-pulse technique for the transient absorption measurement, and the observations of formation processes and ultrafast reactions of hydrated electrons in water. Finally, two innovative techniques of EVS and double-decker electron beam accelerator will be introduced.

## **2. Femtosecond pulse radiolysis**

Ultrafast pulse radiolysis is widely constructed with a stroboscopic method (a pump-and-probe method), as shown in **Figure 2**. The electron pulse is used as a radiation pulse, while the analyzing light pulse (laser or Cherenkov light) is used to measure the time evolution of transient absorption of short-lived intermediate species by changing the time interval between the electron pulse and the analyzing light pulse. In the stroboscopic method, the time resolution is determined by the electron pulse duration, the analyzing light pulse duration, and the time jitter between the electron pulse and the analyzing light pulse, as described in Section 3.1.

**Figure 2.** *Schematic of stroboscopic method (pump-and-probe method) in ultrafast pulse radiolysis.*

## *Femtosecond Pulse Radiolysis DOI: http://dx.doi.org/10.5772/intechopen.91691*

It paves the way to observe the short-lived intermediate species and primary

Recently, a technique of laser wakefield electron acceleration instead of the traditional accelerators was applied in ultrafast pulse radiolysis at the Argonne National Laboratory [30]. However, the time resolution is still limited to picosecond because of the electron pulse duration, the time jitter, and the effect due to group velocity mismatch (GVM) between the electron beam and the light in the sample. In order to reduce furthermore the time jitter between the electron pulse and the analyzing light pulse, a technique of a "double-decker electron beam accelerator" was proposed in pulse radiolysis at Osaka University [31, 32]. The entirely synchronized double-decker electron beams with a time interval of 1.4 ns were generated in the photocathode rf gun with 2-ps laser beams, then accelerated to 32 MeV with a booster linac, and finally compressed into femtosecond with a magnetic compressor. One of the double-decker electron beams was used to irradiate the sample, while another beam was converted to the analyzing light, for example, Cherenkov light or terahertz (THz) light, resulting in the generation of entirely synchronized femtosecond-pulsed electron beam and femtosecond analyzing light. Moreover, in order to circumvent the effect of GVM, an innovative technique of "equivalent velocity spectroscopy (EVS)" was developed for pulse radiolysis at Osaka University [33]. The method of ultrafast pulse radiolysis was reviewed in a book entitled "Recent trends in radiation chemistry" edited by Wishart and Rao in 2010 [34]. In this chapter, we describe the details of femtosecond pulse radiolysis, including the generation of femtosecond electron pulses, the synchronization of femtosecond analyzing laser with the electron pulses, double-pulse technique for the transient absorption measurement, and the observations of formation processes and ultrafast reactions of hydrated electrons in water. Finally, two innovative techniques of EVS and double-decker electron beam accelerator will be introduced.

Ultrafast pulse radiolysis is widely constructed with a stroboscopic method (a pump-and-probe method), as shown in **Figure 2**. The electron pulse is used as a radiation pulse, while the analyzing light pulse (laser or Cherenkov light) is used to measure the time evolution of transient absorption of short-lived intermediate species by changing the time interval between the electron pulse and the analyzing light pulse. In the stroboscopic method, the time resolution is determined by the electron pulse duration, the analyzing light pulse duration, and the time jitter between the electron pulse and the analyzing light pulse, as described in Section 3.1.

*Schematic of stroboscopic method (pump-and-probe method) in ultrafast pulse radiolysis.*

processes in radiation chemistry on femtosecond timescale.

*Novel Imaging and Spectroscopy*

**2. Femtosecond pulse radiolysis**

**Figure 2.**

**30**

The progress of the ultrafast laser technology allows us to use a femtosecond laser as an analyzing light source in pulse radiolysis. However, in order to achieve a femtosecond time resolution, three significant techniques are indispensable:


In this section, we focus on the generation of femtosecond electron pulses with a laser photocathode electron gun accelerator, ultrafast pulse radiolysis with the femtosecond-pulsed electron beam and a femtosecond laser, and a double-pulse detection technique to observe the transient absorption in a thin sample cell.

## **2.1 1.6-cell photocathode rf gun**

Femtosecond pulse radiolysis at Osaka University was constructed with a 1.6-cell S-band (2.856 GHz) photocathode rf gun, a booster linac, a magnetic pulse compressor, and an analyzing femtosecond Ti:sapphire laser, as shown in **Figure 3**.

The photocathode rf gun used in femtosecond pulse radiolysis contains two cavities: a half cell and a full cell. The length of the half-cell cavity was equal to 0.6

#### **Figure 3.**

*Schematic of femtosecond pulse radiolysis apparatus at Osaka University, containing a photocathode rf gun, a booster linac, a magnetic pulse compressor, and an analyzing femtosecond Ti:sapphire laser. The rf gun was driven by a picosecond Nd:YLF laser. A time resolution of 240 fs due to the electron beam and the analyzing laser has been achieved [26–29].*

times *λ*/2 to minimize the beam emittance, while *λ* = 104.96 mm is the wavelength of 2.856-GHz rf. The cavities were operated with 10-MW peak rf power to produce a high-peak electric field of 110 MV/m on photocathode and the cavities [35–39]. A copper cathode was used and illuminated by the fourth harmonic of Nd:YLF picosecond laser [262 nm, pulse duration: 5 ps in full width at half maximum (FWHM)], as described in Section 2.4. The incident angle of the laser light was approximately 2° along the electron beam direction using a mirror placed in vacuum. The laser injection phase (gun phase) was adjusted by changing the phase of the reference 79.3-MHz rf signal with a low-power phase shifter. A solenoid magnet was mounted at the exit of the rf gun to reduce the emittance growth due to the space charge effect during the propagation. The maximum magnetic field was 3 kG.

**2.2 Booster linear accelerator**

*DOI: http://dx.doi.org/10.5772/intechopen.91691*

*Femtosecond Pulse Radiolysis*

**2.3 Magnetic pulse compressor**

obtain a short electron pulse [28].

a 98-fs-long electron pulse at 0.17 nC.

**2.4 Laser system**

**33**

the booster linac.

A 2-m-long S-band traveling-wave booster linac was mounted downstream of the rf gun and the solenoid magnet at a distance of 1.2 m from the photocathode. The booster linac was driven by rf pulses with peak power of 25 MW. The duration of the rf pulses was 4 μs. The repetition rate of the pulses was 10 Hz. The booster linac accelerates the electron pulses and makes an optimum energy-phase correction for pulse compression, where the electrons in the front of pulse has more energy than the electrons in the back of pulse, as shown as the blue pulse in **Figure 3**. The rf phase of the booster linac was adjusted by a high-power phase shifter mounted in the 25-MW rf line. The energy of the electrons was 32 MeV after

The magnetic pulse compressor consists of two 45°-bending magnets and four quadrupole magnets. To obtain an ultrashort electron pulse, all magnets were carefully installed with the minimum lattice error to reduce the aberrations in the phase space distribution. The outside two quadrupole magnets (Q1 and Q4) have equal magnetic fields, while the inside two quadrupole magnets (Q3 and Q4) have equal magnetic fields. They provide the necessary path length dependence on the electron energy. The dispersion function is symmetric on the mid-plane of the compressor. When the picosecond electron pulse with an optimum energy-phase correction (blue pulse in **Figure 3**) passes the magnetic pulse compressor, the pulse is compressed into femtosecond by rotating the pulse in longitudinal phase space, as shown as red pulse in **Figure 3**. However, higher order momentum dispersion in the compressor, especially the second-order dispersion, causes a nonlinear deformation of the longitudinal phase space, yielding the increase of the final pulse duration. To minimize the nonlinear effects, a nonlinear energy correlation of the electron pulse (blue pulse in **Figure 3**) was introduced before the compression by re-phasing the booster linac to >90° with a curvature of the rf waveform. Finally, the correlation offsets the effects of the nonlinear path length in the magnetic compression to

**Figure 5(a)** gives the compressed pulse duration and the relative energy spread as a function of the rf phase of the booster linac at 1 nC [28]. The data show that the shortest pulse duration of 400 58 fs RMS was obtained at 94°. At this phase, the booster linac accelerates the electrons with a nonlinear energy-phase correlation. The pulse duration was decreased by decreasing the electron charge, that is,

The laser system contains an all solid-state LD-pumped Nd:YLF picosecond laser (Time-Bandwidth) and a Ti:sapphire femtosecond laser (Spectra-Physics). The Nd: YLF picosecond laser was used to generate a picosecond electron beam in the rf gun. The laser consists of a 79.3-MHz passive mode-locked Nd:YLF laser oscillator, a regenerative amplifier, and a wavelength converter. The 79.3-MHz laser pulses were phase-locked with a reference 79.3-MHz rf signal, which was generated by dividing the accelerating 2.856-GHz rf by 1/36, by dynamically adjusting the laser cavity length with a semiconductor saturable absorber mirror (SESAM) and a timing stabilizer. A single laser pulse of the oscillator was captured and amplified to the pulse energy up to 1 mJ in the regenerative amplifier. The repetition rate of the amplified pulses was 10 Hz. The amplified laser pulses were converted to ultraviolet

The characteristics of the electron beam generated by the rf gun are shown in **Table 2**. **Figure 4** gives the transverse emittance, pulse duration, and relative energy spread in root-mean-square (RMS) as a function of the laser injection phase. The rf gun enables to generate a low-emittance and low-energy-spread electron beam at the laser injection phase of <40°. Moreover, the pulse compression inside the rf gun due to the rf effect occurred at <40°, yielding a electron pulse duration less than that of the incident laser. However, the electron charge was decreased at a low laser injection phase, that is, 1.8 nC at 80° was decreased to 0.2 nC at 10°. In the pulse radiolysis experiment, the gun phase was fixed to 30° in the experiments.


**Table 2.**

*The characteristics of picosecond electron beam generated by the rf gun at the laser injection phase of 30° [35].*

#### **Figure 4.**

*The transverse emittance, pulse duration, and relative energy spread as a function of the laser injection phase [35]. The electron charge was 1.8 nC at 80° and was decreased to 0.2 nC at 10°.*

## **2.2 Booster linear accelerator**

times *λ*/2 to minimize the beam emittance, while *λ* = 104.96 mm is the wavelength of 2.856-GHz rf. The cavities were operated with 10-MW peak rf power to produce a high-peak electric field of 110 MV/m on photocathode and the cavities [35–39]. A copper cathode was used and illuminated by the fourth harmonic of Nd:YLF picosecond laser [262 nm, pulse duration: 5 ps in full width at half maximum (FWHM)], as described in Section 2.4. The incident angle of the laser light was approximately 2° along the electron beam direction using a mirror placed in vacuum. The laser injection phase (gun phase) was adjusted by changing the phase of the reference 79.3-MHz rf signal with a low-power phase shifter. A solenoid magnet was mounted at the exit of the rf gun to reduce the emittance growth due to the space charge effect during the propagation. The maximum magnetic field

The characteristics of the electron beam generated by the rf gun are shown in **Table 2**. **Figure 4** gives the transverse emittance, pulse duration, and relative energy spread in root-mean-square (RMS) as a function of the laser injection phase. The rf gun enables to generate a low-emittance and low-energy-spread electron beam at the laser injection phase of <40°. Moreover, the pulse compression inside the rf gun due to the rf effect occurred at <40°, yielding a electron pulse duration less than that of the incident laser. However, the electron charge was decreased at a low laser injection phase, that is, 1.8 nC at 80° was decreased to 0.2 nC at 10°. In the pulse radiolysis experiment, the gun phase was fixed to 30° in the experiments.

Electron energy 4 MeV Electron charge 1 nC/pulse Pulse duration 1.8 0.2 ps Transverse emittance 3.2 0.2 mm-mrad at 1 nC

Relative energy spread 0.05% Repetition rate of pulses 10 Hz

*The characteristics of picosecond electron beam generated by the rf gun at the laser injection phase of 30° [35].*

*The transverse emittance, pulse duration, and relative energy spread as a function of the laser injection phase*

*[35]. The electron charge was 1.8 nC at 80° and was decreased to 0.2 nC at 10°.*

was 3 kG.

*Novel Imaging and Spectroscopy*

**Table 2.**

**Figure 4.**

**32**

A 2-m-long S-band traveling-wave booster linac was mounted downstream of the rf gun and the solenoid magnet at a distance of 1.2 m from the photocathode. The booster linac was driven by rf pulses with peak power of 25 MW. The duration of the rf pulses was 4 μs. The repetition rate of the pulses was 10 Hz. The booster linac accelerates the electron pulses and makes an optimum energy-phase correction for pulse compression, where the electrons in the front of pulse has more energy than the electrons in the back of pulse, as shown as the blue pulse in **Figure 3**. The rf phase of the booster linac was adjusted by a high-power phase shifter mounted in the 25-MW rf line. The energy of the electrons was 32 MeV after the booster linac.

## **2.3 Magnetic pulse compressor**

The magnetic pulse compressor consists of two 45°-bending magnets and four quadrupole magnets. To obtain an ultrashort electron pulse, all magnets were carefully installed with the minimum lattice error to reduce the aberrations in the phase space distribution. The outside two quadrupole magnets (Q1 and Q4) have equal magnetic fields, while the inside two quadrupole magnets (Q3 and Q4) have equal magnetic fields. They provide the necessary path length dependence on the electron energy. The dispersion function is symmetric on the mid-plane of the compressor. When the picosecond electron pulse with an optimum energy-phase correction (blue pulse in **Figure 3**) passes the magnetic pulse compressor, the pulse is compressed into femtosecond by rotating the pulse in longitudinal phase space, as shown as red pulse in **Figure 3**. However, higher order momentum dispersion in the compressor, especially the second-order dispersion, causes a nonlinear deformation of the longitudinal phase space, yielding the increase of the final pulse duration. To minimize the nonlinear effects, a nonlinear energy correlation of the electron pulse (blue pulse in **Figure 3**) was introduced before the compression by re-phasing the booster linac to >90° with a curvature of the rf waveform. Finally, the correlation offsets the effects of the nonlinear path length in the magnetic compression to obtain a short electron pulse [28].

**Figure 5(a)** gives the compressed pulse duration and the relative energy spread as a function of the rf phase of the booster linac at 1 nC [28]. The data show that the shortest pulse duration of 400 58 fs RMS was obtained at 94°. At this phase, the booster linac accelerates the electrons with a nonlinear energy-phase correlation. The pulse duration was decreased by decreasing the electron charge, that is, a 98-fs-long electron pulse at 0.17 nC.

### **2.4 Laser system**

The laser system contains an all solid-state LD-pumped Nd:YLF picosecond laser (Time-Bandwidth) and a Ti:sapphire femtosecond laser (Spectra-Physics). The Nd: YLF picosecond laser was used to generate a picosecond electron beam in the rf gun. The laser consists of a 79.3-MHz passive mode-locked Nd:YLF laser oscillator, a regenerative amplifier, and a wavelength converter. The 79.3-MHz laser pulses were phase-locked with a reference 79.3-MHz rf signal, which was generated by dividing the accelerating 2.856-GHz rf by 1/36, by dynamically adjusting the laser cavity length with a semiconductor saturable absorber mirror (SESAM) and a timing stabilizer. A single laser pulse of the oscillator was captured and amplified to the pulse energy up to 1 mJ in the regenerative amplifier. The repetition rate of the amplified pulses was 10 Hz. The amplified laser pulses were converted to ultraviolet

parametric amplifier to extend the tuning range into ultraviolet (UV), visible (VIS), or infrared (IR). **Figure 6(b)** shows the pulse energies of outputs of TOPAS-Prime and NirUVis [40]. An analyzing light with a wide wavelength range of 190�1600 nm facilitates pulse radiolysis based on the sample's requirements. The time delay between the electron pulse and the analyzing laser pulse was adjusted by changing

Progress in the photodiode technology allows us to easily detect the transient absorption of intermediate species over a wide wavelength range from ultraviolet to infrared, that is, using silicon photodetectors in wavelength range from 200 to 1000 nm (PDA10A, Thorlabs) and InGaAs photodetectors in wavelength range from 800 to 1700 nm (PDA10C, Thorlabs). However, the development of transient absorption detection technique with a good S/N ratio is very significant in ultrafast pulse radiolysis, especially for the use of low-charge femtosecond electron pulses

For this purpose, we developed a double-pulse measurement technique to reduce the fluctuation of the laser intensity caused by long-term drift and mechanical vibration of optics. **Figure 7** shows the concept of the double-pulse measurement. Two analyzing laser pulses with a time interval of *Δt* are incident on the sample. The front laser pulse is used as "reference pulse" before the electron irradiation, while the back laser pulse is used as a "signal pulse" to measure the absorption

*OD* <sup>¼</sup> log *<sup>I</sup>*<sup>0</sup>

where *I0* and *I* are the signals of the reference and signal pulses detected by a photodiode, respectively. The time interval between two pulses was *Δt* = 1 ms using

The output of the Ti:sapphire laser oscillator (Tsunami) with the wavelength of 800 nm was also utilized in pulse radiolysis [29]. The continuous pulses of 79.3 MHz were guided to a pulse selector (Spectra-Physics, 3980), which was constructed with an Acousto-Optic Modulator (AOM) crystal driven by the 79.3-MHz rf pulses. The selector extracted a pulse train containing several pulses by adjusting the rf pulse duration and phase. Finally, two stable pulses of the pulse train with a same

*Schematic of double-pulse measurement in pulse radiolysis to reduce the fluctuation of the laser intensity caused*

*<sup>I</sup>* (5)

after the electron irradiation. The optical density is thus obtained by

the arrival time of the laser pulse at the sample with an optical delay.

*Femtosecond Pulse Radiolysis*

*DOI: http://dx.doi.org/10.5772/intechopen.91691*

**2.5 Double-pulse technique: transient absorption measurement**

and a thin sample cell to circumvent the effect of GVM.

the outputs of the optical parametric amplifier.

*by long-term drift and mechanical vibration of optics.*

**Figure 7.**

**35**

**Figure 5.**

*(a) The compressed pulse duration and the relative energy spread as a function of the rf phase of booster linac at 1 nC and (b) the compressed pulse duration as a function of electron charge at the rf phase of 94° [28]. Copyright 2006, with permission from Elsevier.*

pulses (UV: 262 nm) using two nonlinear crystals and guided to the photocathode in the rf gun. The pulse duration of UV light was 5 ps FWHM. The maximum pulse energy was 300 μJ.

The mode-locked Ti:sapphire femtosecond laser (Spectra-Physics) was used as analyzing light source in pulse radiolysis. The laser consists of a 79.3-MHz femtosecond Ti:sapphire laser oscillator (Tsunami, central wavelength: 800 nm), a regenerative amplifier (Spitifire), and a tunable optical parametric amplifier with frequency mixer stages (TOPAS-Prime and NirUVis), as shown in **Figure 6(a)**. The 79.3-MHz laser pulses were synchronized to the external 79.3-MHz RF signal with a time-to-lock piezoelectric device. The time jitter between the laser pulse and 79.3- MHz rf phase was 61 fs RMS [26], being approximately equal to the time jitter between the electron pulse and the analyzing laser pulse. The laser oscillator output was fed to the regenerative amplifier for pulse stretching, amplification, and compression. The regenerative amplifier was driven by a green laser with a repetition rate of 1 kHz (Empower, wavelength: 532 nm, output: 15 W). The pulse energy of the amplifier output was 0.8 mJ. The pulse duration was 100 fs after the pulse compression. The amplified femtosecond laser beam was inputted to the optical

#### **Figure 6.**

*(a) Photo of TOPAS-Prime and NirUVis and (b) the pulse energy of analyzing light as a function of wavelength, which facilitates pulse radiolysis [40]. Copyright 2017, with permission from Japan Radioisotope Association.*

### *Femtosecond Pulse Radiolysis DOI: http://dx.doi.org/10.5772/intechopen.91691*

parametric amplifier to extend the tuning range into ultraviolet (UV), visible (VIS), or infrared (IR). **Figure 6(b)** shows the pulse energies of outputs of TOPAS-Prime and NirUVis [40]. An analyzing light with a wide wavelength range of 190�1600 nm facilitates pulse radiolysis based on the sample's requirements. The time delay between the electron pulse and the analyzing laser pulse was adjusted by changing the arrival time of the laser pulse at the sample with an optical delay.

## **2.5 Double-pulse technique: transient absorption measurement**

Progress in the photodiode technology allows us to easily detect the transient absorption of intermediate species over a wide wavelength range from ultraviolet to infrared, that is, using silicon photodetectors in wavelength range from 200 to 1000 nm (PDA10A, Thorlabs) and InGaAs photodetectors in wavelength range from 800 to 1700 nm (PDA10C, Thorlabs). However, the development of transient absorption detection technique with a good S/N ratio is very significant in ultrafast pulse radiolysis, especially for the use of low-charge femtosecond electron pulses and a thin sample cell to circumvent the effect of GVM.

For this purpose, we developed a double-pulse measurement technique to reduce the fluctuation of the laser intensity caused by long-term drift and mechanical vibration of optics. **Figure 7** shows the concept of the double-pulse measurement. Two analyzing laser pulses with a time interval of *Δt* are incident on the sample. The front laser pulse is used as "reference pulse" before the electron irradiation, while the back laser pulse is used as a "signal pulse" to measure the absorption after the electron irradiation. The optical density is thus obtained by

$$OD = \log \frac{I\_0}{I} \tag{5}$$

where *I0* and *I* are the signals of the reference and signal pulses detected by a photodiode, respectively. The time interval between two pulses was *Δt* = 1 ms using the outputs of the optical parametric amplifier.

The output of the Ti:sapphire laser oscillator (Tsunami) with the wavelength of 800 nm was also utilized in pulse radiolysis [29]. The continuous pulses of 79.3 MHz were guided to a pulse selector (Spectra-Physics, 3980), which was constructed with an Acousto-Optic Modulator (AOM) crystal driven by the 79.3-MHz rf pulses. The selector extracted a pulse train containing several pulses by adjusting the rf pulse duration and phase. Finally, two stable pulses of the pulse train with a same

#### **Figure 7.**

pulses (UV: 262 nm) using two nonlinear crystals and guided to the photocathode in the rf gun. The pulse duration of UV light was 5 ps FWHM. The maximum pulse

*(a) The compressed pulse duration and the relative energy spread as a function of the rf phase of booster linac at 1 nC and (b) the compressed pulse duration as a function of electron charge at the rf phase of 94° [28].*

The mode-locked Ti:sapphire femtosecond laser (Spectra-Physics) was used as analyzing light source in pulse radiolysis. The laser consists of a 79.3-MHz femtosecond Ti:sapphire laser oscillator (Tsunami, central wavelength: 800 nm), a regenerative amplifier (Spitifire), and a tunable optical parametric amplifier with frequency mixer stages (TOPAS-Prime and NirUVis), as shown in **Figure 6(a)**. The 79.3-MHz laser pulses were synchronized to the external 79.3-MHz RF signal with a time-to-lock piezoelectric device. The time jitter between the laser pulse and 79.3- MHz rf phase was 61 fs RMS [26], being approximately equal to the time jitter between the electron pulse and the analyzing laser pulse. The laser oscillator output was fed to the regenerative amplifier for pulse stretching, amplification, and compression. The regenerative amplifier was driven by a green laser with a repetition rate of 1 kHz (Empower, wavelength: 532 nm, output: 15 W). The pulse energy of the amplifier output was 0.8 mJ. The pulse duration was 100 fs after the pulse compression. The amplified femtosecond laser beam was inputted to the optical

*(a) Photo of TOPAS-Prime and NirUVis and (b) the pulse energy of analyzing light as a function of wavelength, which facilitates pulse radiolysis [40]. Copyright 2017, with permission from Japan Radioisotope*

energy was 300 μJ.

*Copyright 2006, with permission from Elsevier.*

*Novel Imaging and Spectroscopy*

**Figure 5.**

**Figure 6.**

*Association.*

**34**

*Schematic of double-pulse measurement in pulse radiolysis to reduce the fluctuation of the laser intensity caused by long-term drift and mechanical vibration of optics.*

intensity were used as the double-pulse measurement. The time interval between two pulses was *Δt* = 12.6 ns. The previous studies [26, 27] indicate that this technique enables to detect a small optical density of 0.001 with an acceptable S/N ratio in a thin sample cell with optical length of 0.18 mm.

where *e* �

*Femtosecond Pulse Radiolysis*

*DOI: http://dx.doi.org/10.5772/intechopen.91691*

estimated by

OH<sup>∙</sup> radical pair:

**37**

pulses, yielding *δtb* = 240 fs.

shown in **Figure 7** can be calculated by

constant of 550 � 50 fs in water as shown in **Figure 8**.

*e* �

hydrated electrons in water as in the photolysis study [41]:

*e* �

*th* represents the thermalized electron. When we fit the data in **Figure 8(a)** with the reaction (6), we found that the hydrated electrons are formed by the decay of pre-hydrated electrons with a time constant of *τ*<sup>2</sup> = 550 � 50 fs and the pre-hydrated electrons are formed within *τ*<sup>1</sup> = 110�200 fs after the electron irradiation. The obtained formation time of hydrated electrons in water pulse radiolysis is in agreement with that of *τ*<sup>1</sup> = 540 fs obtained in multiphoton ionization studies [2, 3]. The time resolution of pulse radiolysis based on the stroboscopic method can be estimated by two components: one is the time resolution due to the electron beam and the analyzing laser light, and another is the time resolution due to GVM in the sample. The time resolution due to the electron beam and the laser light can be

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

*<sup>l</sup>* <sup>þ</sup> *<sup>σ</sup>*<sup>2</sup> *j*

� � (8)

*hyd* þ H3O<sup>þ</sup> ! H2O þ H (9)

*hyd* þ OH� ! OH‐ (10)

(7)

*σ*2 *<sup>e</sup>* <sup>þ</sup> *<sup>σ</sup>*<sup>2</sup>

where *σ<sup>e</sup>* = 201 fs RMS is the electron pulse duration, *σ<sup>l</sup>* = 107 fs RMS is the analyzing laser pulse duration, and *σ<sup>j</sup>* = 61 fs RMS is the time jitter between the two

The time resolution due to GVM in the sample with the experimental setup as

*c* � 1 *v*

where *L* = 0.18 mm is the optical length of the sample, *n* is the refractive index of the sample, *n* = 1.33 for water sample, *c* is the velocity of light in vacuum, and *v* is the velocity of the electrons (*v* ffi *c* for the 32-MeV electrons), yielding *g(L)* = 198 fs. Therefore, the total resolution of pulse radiation was *Δτ* = *δtb* + *g(L)* = 438 fs, which is available to observe the formation process of hydrated electrons with a time

Although the formation time of hydrated electrons in pulse radiolysis is similar to that in the multiphoton ionization, the thermalization distance (length of initial distribution) of electrons in pulse radiolysis is longer than that in the photoionization because the electrons produced by radiation ionization have a high initial kinetic energy. It causes some difference of spur reactions (primary processes) in the early time, containing the reactions of hydrated electrons with H3O<sup>+</sup> cation and

**Figure 9** shows the time evolution of hydrated electrons observed in water pulse radiolysis at 800 nm [40] and obtained in photolysis with a photoexcitation energy of 8.3 eV [41]. The data show that a small magnitude (�10%) of the hydrated electrons decreases in the first 40 ps due to the spur reactions (reactions (9) and (10)) in pulse radiolysis, while 38% of hydrated electrons reacted with H3O+ cation and OH<sup>∙</sup> radical pair in photolysis with a photoexcitation energy of 8.3 eV. We fit the data by a numerical calculation of the survival probability of the hydrated electrons in the reactions (9) and (10) with a Gaussian initial distribution of

*g L*ð Þ¼ *<sup>L</sup> <sup>n</sup>*

**3.2 Observations of ultrafast spur reactions in water pulse radiolysis**

q

*δtb* ¼

## **3. Pulse radiolysis experiments**

### **3.1 Observations of solvation dynamics of hydrated electrons in water**

In order to reduce the degradation of time resolution due to GVM, a thin sample cell with an optical length of 0.18 mm was used in the observation of hydrated electrons [26]. The water sample was deionized and Ar-saturated before the measurement. The double pulses with time interval of 12.6 ns generated by the 79.3- MHz Ti:sapphire laser oscillator and the pulse selector were used as analyzing light. The wavelength was 800 nm with a bandwidth of 12.5 nm FWHM. The pulse energy was approximately 10 nJ. The duration of the laser pulses was 107 fs RMS. The intensities of the reference pulse and the signal pulse were detected by a silicon photodiode and a digital phosphor oscilloscope. A femtosecond electron beam with pulse duration of 201 fs RMS at 0.4 nC generated by the accelerator system was used to irradiate the sample. The time jitter between the laser pulse and the electron pulse was measured to 61 fs RMS.

**Figure 8** shows the first observation of ultrafast transient absorption kinetics of hydrated and pre-hydrated electrons in water pulse radiolysis. The transient absorption kinetics of pre-hydrated electrons were observed in a water sample cell with an optical length of 1 mm at the wavelength of 1300 nm. The kinetics observed in **Figure 8(a)** are in agreement with the formation process of hydrated electrons in the multiphoton ionization with two-state model, in which the hydrated electron is formed via a shortlived precursor (called "pre-hydrated electron," denoted by *e* � *pre*) as

*e*

$$
\bar{e}\_{th}^- \xrightarrow{\tau\_1} \bar{e}\_{pre}^- \xrightarrow{\tau\_2} \bar{e}\_{hyd}^- \tag{6}
$$

#### **Figure 8.**

*(a) Femtosecond transient absorption kinetics of hydrated electrons observed in a thin water sample with an optical length of 0.18 mm at the wavelength of 800 nm, and (b) picosecond transient absorption kinetics of pre-hydrated electrons observed in a water sample cell with an optical length of 1 mm at the wavelength of 1300 nm. The blue and green lines in Figure 8(a) show the decay of pre-hydrated electrons and the formation of hydrated electrons with a time constant of 550 fs. The black line represents the kinetics of reaction (6) with* τ*<sup>1</sup> = 180 fs and* τ*<sup>2</sup> = 550* � *50 fs. The data at* ≤ *3 ps in Figure 8(b) are mainly contributed by the pre-hydrated electrons, while the data at > 3 ps represent the kinetics of hydrated electrons with a long time decay. Copyright 2017, with permission from Japan Radioisotope Association.*

*Femtosecond Pulse Radiolysis DOI: http://dx.doi.org/10.5772/intechopen.91691*

intensity were used as the double-pulse measurement. The time interval between two pulses was *Δt* = 12.6 ns. The previous studies [26, 27] indicate that this technique enables to detect a small optical density of 0.001 with an acceptable S/N ratio

**3.1 Observations of solvation dynamics of hydrated electrons in water**

cell with an optical length of 0.18 mm was used in the observation of hydrated electrons [26]. The water sample was deionized and Ar-saturated before the measurement. The double pulses with time interval of 12.6 ns generated by the 79.3- MHz Ti:sapphire laser oscillator and the pulse selector were used as analyzing light. The wavelength was 800 nm with a bandwidth of 12.5 nm FWHM. The pulse energy was approximately 10 nJ. The duration of the laser pulses was 107 fs RMS. The intensities of the reference pulse and the signal pulse were detected by a silicon photodiode and a digital phosphor oscilloscope. A femtosecond electron beam with pulse duration of 201 fs RMS at 0.4 nC generated by the accelerator system was used to irradiate the sample. The time jitter between the laser pulse and the electron

**Figure 8** shows the first observation of ultrafast transient absorption kinetics of hydrated and pre-hydrated electrons in water pulse radiolysis. The transient absorption kinetics of pre-hydrated electrons were observed in a water sample cell with an optical length of 1 mm at the wavelength of 1300 nm. The kinetics observed in **Figure 8(a)** are in agreement with the formation process of hydrated electrons in the multiphoton ionization with two-state model, in which the hydrated electron is formed via a short-

*(a) Femtosecond transient absorption kinetics of hydrated electrons observed in a thin water sample with an optical length of 0.18 mm at the wavelength of 800 nm, and (b) picosecond transient absorption kinetics of pre-hydrated electrons observed in a water sample cell with an optical length of 1 mm at the wavelength of 1300 nm. The blue and green lines in Figure 8(a) show the decay of pre-hydrated electrons and the formation of hydrated electrons with a time constant of 550 fs. The black line represents the kinetics of reaction (6) with* τ*<sup>1</sup> = 180 fs and* τ*<sup>2</sup> = 550* � *50 fs. The data at* ≤ *3 ps in Figure 8(b) are mainly contributed by the pre-hydrated electrons, while the data at > 3 ps represent the kinetics of hydrated electrons with a long time decay. Copyright*

� *pre*) as

*hyd* (6)

lived precursor (called "pre-hydrated electron," denoted by *e*

*2017, with permission from Japan Radioisotope Association.*

*e* � *th* ! *τ*1 *e* � *pre* ! *τ*2 *e* �

In order to reduce the degradation of time resolution due to GVM, a thin sample

in a thin sample cell with optical length of 0.18 mm.

**3. Pulse radiolysis experiments**

*Novel Imaging and Spectroscopy*

pulse was measured to 61 fs RMS.

**Figure 8.**

**36**

where *e* � *th* represents the thermalized electron. When we fit the data in **Figure 8(a)** with the reaction (6), we found that the hydrated electrons are formed by the decay of pre-hydrated electrons with a time constant of *τ*<sup>2</sup> = 550 � 50 fs and the pre-hydrated electrons are formed within *τ*<sup>1</sup> = 110�200 fs after the electron irradiation. The obtained formation time of hydrated electrons in water pulse radiolysis is in agreement with that of *τ*<sup>1</sup> = 540 fs obtained in multiphoton ionization studies [2, 3].

The time resolution of pulse radiolysis based on the stroboscopic method can be estimated by two components: one is the time resolution due to the electron beam and the analyzing laser light, and another is the time resolution due to GVM in the sample. The time resolution due to the electron beam and the laser light can be estimated by

$$
\delta t\_b = \sqrt{\sigma\_\epsilon^2 + \sigma\_l^2 + \sigma\_j^2} \tag{7}
$$

where *σ<sup>e</sup>* = 201 fs RMS is the electron pulse duration, *σ<sup>l</sup>* = 107 fs RMS is the analyzing laser pulse duration, and *σ<sup>j</sup>* = 61 fs RMS is the time jitter between the two pulses, yielding *δtb* = 240 fs.

The time resolution due to GVM in the sample with the experimental setup as shown in **Figure 7** can be calculated by

$$\mathbf{g}(L) = L\left(\frac{n}{\mathcal{c}} - \frac{1}{\mathcal{v}}\right) \tag{8}$$

where *L* = 0.18 mm is the optical length of the sample, *n* is the refractive index of the sample, *n* = 1.33 for water sample, *c* is the velocity of light in vacuum, and *v* is the velocity of the electrons (*v* ffi *c* for the 32-MeV electrons), yielding *g(L)* = 198 fs. Therefore, the total resolution of pulse radiation was *Δτ* = *δtb* + *g(L)* = 438 fs, which is available to observe the formation process of hydrated electrons with a time constant of 550 � 50 fs in water as shown in **Figure 8**.

#### **3.2 Observations of ultrafast spur reactions in water pulse radiolysis**

Although the formation time of hydrated electrons in pulse radiolysis is similar to that in the multiphoton ionization, the thermalization distance (length of initial distribution) of electrons in pulse radiolysis is longer than that in the photoionization because the electrons produced by radiation ionization have a high initial kinetic energy. It causes some difference of spur reactions (primary processes) in the early time, containing the reactions of hydrated electrons with H3O<sup>+</sup> cation and OH<sup>∙</sup> radical pair:

$$\text{H}\_{hyd}^- + \text{H}\_3\text{O}^+ \rightarrow \text{H}\_2\text{O} + \text{H} \tag{9}$$

$$e^-\_{hyd} + \text{OH} \cdot \rightarrow \text{OH}^\cdot \tag{10}$$

**Figure 9** shows the time evolution of hydrated electrons observed in water pulse radiolysis at 800 nm [40] and obtained in photolysis with a photoexcitation energy of 8.3 eV [41]. The data show that a small magnitude (�10%) of the hydrated electrons decreases in the first 40 ps due to the spur reactions (reactions (9) and (10)) in pulse radiolysis, while 38% of hydrated electrons reacted with H3O+ cation and OH<sup>∙</sup> radical pair in photolysis with a photoexcitation energy of 8.3 eV. We fit the data by a numerical calculation of the survival probability of the hydrated electrons in the reactions (9) and (10) with a Gaussian initial distribution of hydrated electrons in water as in the photolysis study [41]:

**Figure 9.**

*Time evolution of hydrated electrons observed in water pulse radiolysis at 800 nm (blue lines) and in photolysis with a photoexcitation energy of 8.3 eV (green line) [41]. Copyright 2017, with permission from Japan Radioisotope Association.*

$$f(r\_0) = \frac{1}{\sqrt{8\pi^3 \sigma^6}} \exp\left(-\frac{r\_0^2}{2\sigma^2}\right), \quad \text{and} \quad \langle r\_0 \rangle = \sigma \sqrt{8/\pi} \tag{11}$$

In EVS, the time resolution due to GVM in the sample can be thus calculated by

*Schematic of EVS in pulse radiolysis with rotated electron pulses and analyzing laser pulses [33]. Copyright*

1 *ve*sin<sup>θ</sup> � cos<sup>θ</sup> *vl*sinθ for L<sup>&</sup>gt; *<sup>σ</sup>l*cos<sup>θ</sup>

The technique of the electron pulse rotation is very significant in EVS. In order

to rotate the electron pulse, first, we adjusted the accelerating rf phase in the booster linac to increase the relative energy spread in the electron pulse, shown as the red pulse at the exit of the linac in **Figure 11(a)**. Then, we transported the electron pulse through the magnetic pulse compressor. The electron pulse was thus rotated along the propagation direction, shown as the red pulse at the exit of the compressor in **Figure 11(a)**. The rotation angle increased by increasing the accelerating rf phase in the booster linac. **Figure 11(b)** shows the 2D images of the electron pulses with different rotation angle at exit of the pulse compressor measured by a femtosecond streak camera at the accelerating rf phase of ϕrf = 94°, 99°, and 103° [33]. The electron charge was 1.8 nC per pulse. At ϕrf = 94°, an optimal energyphase correlation in the electron pulse was produced for the bunch compressor, as described in Section 2.3. The electron pulse was compressed into the shortest pulse duration of 2 ps FWHM or 0.8 ps RMS, but the electron pulse was not rotated. At ϕrf = 99°, the rotation angle of the electron pulse was φ = 41°, which is a required angle for water sample. However, the pulse duration was increased to 5 ps. It was caused mainly by the large relative energy spread in the magnetic pulse compressor.

At ϕrf = 103°, φ= 59°, and the pulse duration was increased to 6 ps.

In the demonstration of EVS, we measured the transient absorption kinetics of hydrated electrons in water using rotated electron pulses (φ = 41°) and unrotated electron pulses (φ = 0°). A sample cell with an optical length of 10 mm was used. The electron beam and the analyzing light were incident on the sample with the angle of θ = 41°. The electron charge was 1.8 nC per pulse. The duration of the

where *σ<sup>e</sup>* and *σ<sup>l</sup>* are the sizes of the electron beam and the analyzing light respectively,φ is the rotation angle of the electron pulse, *ve* and *vl* are the velocities of the electrons and the analyzing light in the sample respectively, with≈ *c* for the 32-MeV electrons and *vl* ¼ *c=n*,. If φ ¼ θ in Eq. (12), g Lð Þ¼ 0. It means that the resolution limitation due to GVM in the sample is thus fully removed. Moreover, a

sin<sup>θ</sup> <sup>þ</sup> *<sup>σ</sup><sup>e</sup>* sinθ

, (12)

g Lð Þ¼ *σ<sup>e</sup>*

**39**

**Figure 10.**

sinφ *ve*cosφ

*2009, with permission from Elsevier.*

*Femtosecond Pulse Radiolysis*

*DOI: http://dx.doi.org/10.5772/intechopen.91691*

<sup>þ</sup> cos<sup>θ</sup> *ve*sin<sup>θ</sup> � <sup>1</sup>

long optical length (L) can be used in EVS.

*vl*sinθ

þ *σ<sup>l</sup>*

where h*r*0i is the distance of the hydrated electron distribution (the average thermalization distance). We found h i *r*<sup>0</sup> = 7�9 nm in pulse radiolysis, which is much longer than h*r*0i = 0.9 nm in the photolysis with a photoexcitation energy of 8.3 eV. The long distance in water pulse radiolysis causes a small magnitude of the hydrated electrons reacting with H3O<sup>+</sup> cation and OH<sup>∙</sup> radical pair in the first 40 ps.

The raw data in **Figure 9** give the ratio of optical density at 1.5 and 20ps, OD1.5ps/OD20ps = 1.12. Using a hydrated electron yield (G-value) of 4.1 per 100 eV obtained at 20ps in previous water pulse radiolysis studies [21, 22], we then obtained G = 4.6 � 0.3 at 1.5ps, which is as good as the initial yield of hydrated electrons in water pulse radiolysis because the thermalized electrons are fully hydrated at 1.5ps. The obtained G-value is in agreement with the initial yield of 4.8 estimated in scavenger studies [42].

#### **3.3 Equivalent velocity spectroscopy**

The degradation of time resolution due to GVM in the sample can be reduced using a thin sample cell. However, the use of thin sample cell leads to a small absorption signal (optical intensity) and the degradation of S/N ratio in the measurement. On the other hand, due to the space charge effect, the ultrashort electron pulses are realized only at low electron charge, that is, sub-20-fs electron pulses at 2.1 pC [39]. Therefore, the increase of both the time resolution and the absorption signal is a great challenge in pulse radiolysis.

To circumvent the GVM problem and increase the absorption signal, we proposed an innovative technique of equivalent velocity spectroscopy (EVS) in pulse radiolysis [33]. The approach of EVS is shown in **Figure 10**. The electron pulse and the analyzing laser pulse are incident on the sample with an angle (θ) determined by the refractive index (*n*) of the sample as cosθ = 1/*n*. If we rotate the electron pulse with an angle (φ) and φ ¼ θ, both the rotated electron pulse and the analyzing light pulse precisely overlap at every point in the sample, resulting in the entire circumvention of GVM between the electron beam and the light in the sample.

*Femtosecond Pulse Radiolysis DOI: http://dx.doi.org/10.5772/intechopen.91691*

#### **Figure 10.**

*f r*ð Þ¼ <sup>0</sup>

**Figure 9.**

*Radioisotope Association.*

*Novel Imaging and Spectroscopy*

estimated in scavenger studies [42].

**3.3 Equivalent velocity spectroscopy**

signal is a great challenge in pulse radiolysis.

the sample.

**38**

1 ffiffiffiffiffiffiffiffiffiffiffiffi

<sup>8</sup>*π*<sup>3</sup>*σ*<sup>6</sup> <sup>p</sup> exp � *<sup>r</sup>*<sup>2</sup>

0 2*σ*<sup>2</sup> � �

*Time evolution of hydrated electrons observed in water pulse radiolysis at 800 nm (blue lines) and in photolysis with a photoexcitation energy of 8.3 eV (green line) [41]. Copyright 2017, with permission from Japan*

where h*r*0i is the distance of the hydrated electron distribution (the average thermalization distance). We found h i *r*<sup>0</sup> = 7�9 nm in pulse radiolysis, which is much longer than h*r*0i = 0.9 nm in the photolysis with a photoexcitation energy of 8.3 eV. The long distance in water pulse radiolysis causes a small magnitude of the hydrated electrons reacting with H3O<sup>+</sup> cation and OH<sup>∙</sup> radical pair in the first 40 ps. The raw data in **Figure 9** give the ratio of optical density at 1.5 and 20ps, OD1.5ps/OD20ps = 1.12. Using a hydrated electron yield (G-value) of 4.1 per 100 eV obtained at 20ps in previous water pulse radiolysis studies [21, 22], we then obtained G = 4.6 � 0.3 at 1.5ps, which is as good as the initial yield of hydrated electrons in water pulse radiolysis because the thermalized electrons are fully hydrated at 1.5ps. The obtained G-value is in agreement with the initial yield of 4.8

The degradation of time resolution due to GVM in the sample can be reduced using a thin sample cell. However, the use of thin sample cell leads to a small absorption signal (optical intensity) and the degradation of S/N ratio in the measurement. On the other hand, due to the space charge effect, the ultrashort electron pulses are realized only at low electron charge, that is, sub-20-fs electron pulses at 2.1 pC [39]. Therefore, the increase of both the time resolution and the absorption

To circumvent the GVM problem and increase the absorption signal, we proposed an innovative technique of equivalent velocity spectroscopy (EVS) in pulse radiolysis [33]. The approach of EVS is shown in **Figure 10**. The electron pulse and the analyzing laser pulse are incident on the sample with an angle (θ) determined by the refractive index (*n*) of the sample as cosθ = 1/*n*. If we rotate the electron pulse with an angle (φ) and φ ¼ θ, both the rotated electron pulse and the analyzing light pulse precisely overlap at every point in the sample, resulting in the entire circumvention of GVM between the electron beam and the light in

, and h i *<sup>r</sup>*<sup>0</sup> <sup>¼</sup> *<sup>σ</sup>* ffiffiffiffiffiffiffiffi

8*=π* p (11)

*Schematic of EVS in pulse radiolysis with rotated electron pulses and analyzing laser pulses [33]. Copyright 2009, with permission from Elsevier.*

In EVS, the time resolution due to GVM in the sample can be thus calculated by

$$\log(\mathcal{L}) = \sigma\_{\varepsilon} \left( \frac{\sin \phi}{\upsilon\_{\varepsilon} \cos \phi} + \frac{\cos \theta}{\upsilon\_{\varepsilon} \sin \theta} - \frac{1}{\upsilon\_{l} \sin \theta} \right) + \sigma\_{l} \left( \frac{1}{\upsilon\_{\varepsilon} \sin \theta} - \frac{\cos \theta}{\upsilon\_{l} \sin \theta} \right) \text{ for } \mathcal{L} > \frac{\sigma\_{l} \cos \theta}{\sin \theta} + \frac{\sigma\_{\varepsilon}}{\sin \theta}, \tag{12}$$

where *σ<sup>e</sup>* and *σ<sup>l</sup>* are the sizes of the electron beam and the analyzing light respectively,φ is the rotation angle of the electron pulse, *ve* and *vl* are the velocities of the electrons and the analyzing light in the sample respectively, with≈ *c* for the 32-MeV electrons and *vl* ¼ *c=n*,. If φ ¼ θ in Eq. (12), g Lð Þ¼ 0. It means that the resolution limitation due to GVM in the sample is thus fully removed. Moreover, a long optical length (L) can be used in EVS.

The technique of the electron pulse rotation is very significant in EVS. In order to rotate the electron pulse, first, we adjusted the accelerating rf phase in the booster linac to increase the relative energy spread in the electron pulse, shown as the red pulse at the exit of the linac in **Figure 11(a)**. Then, we transported the electron pulse through the magnetic pulse compressor. The electron pulse was thus rotated along the propagation direction, shown as the red pulse at the exit of the compressor in **Figure 11(a)**. The rotation angle increased by increasing the accelerating rf phase in the booster linac. **Figure 11(b)** shows the 2D images of the electron pulses with different rotation angle at exit of the pulse compressor measured by a femtosecond streak camera at the accelerating rf phase of ϕrf = 94°, 99°, and 103° [33]. The electron charge was 1.8 nC per pulse. At ϕrf = 94°, an optimal energyphase correlation in the electron pulse was produced for the bunch compressor, as described in Section 2.3. The electron pulse was compressed into the shortest pulse duration of 2 ps FWHM or 0.8 ps RMS, but the electron pulse was not rotated. At ϕrf = 99°, the rotation angle of the electron pulse was φ = 41°, which is a required angle for water sample. However, the pulse duration was increased to 5 ps. It was caused mainly by the large relative energy spread in the magnetic pulse compressor. At ϕrf = 103°, φ= 59°, and the pulse duration was increased to 6 ps.

In the demonstration of EVS, we measured the transient absorption kinetics of hydrated electrons in water using rotated electron pulses (φ = 41°) and unrotated electron pulses (φ = 0°). A sample cell with an optical length of 10 mm was used. The electron beam and the analyzing light were incident on the sample with the angle of θ = 41°. The electron charge was 1.8 nC per pulse. The duration of the

**3.4 Double-decker electron beam accelerator**

compressor.

**41**

**Figure 12.**

*Femtosecond Pulse Radiolysis*

*DOI: http://dx.doi.org/10.5772/intechopen.91691*

Recently, progress of the advanced particle accelerator and ultrashort laser technologies enables the generation of attosecond electron and laser pulses. When using such ultrashort electron and laser pulses in pulse radiolysis, the time jitter between the two pulses becomes a serious problem. Here, we introduce a technology of double-decker electron beam accelerator to generate both entirely synchro-

*Transient absorption kinetics of hydrated electrons in water measured in EVS pulse radiolysis with the rotated electron pulses (blue solid dots) and unrotated electron pulses (red circles) [33]. The wavelength of the*

**Figure 13** shows the concept of the double-decker electron beam accelerator for pulse radiolysis [31, 32]. The output of Nd:YLF picosecond laser was divided by a beam splitter to produce 2-ps laser beams with different positions in the vertical direction. The 2-ps laser beams were illuminated the photocathode in the rf gun. An optical delay was mounted in the up beam line to time delay the up beam. Two electron beams generated in the rf gun were accelerated in the booster linac up to 32 MeV and were then compressed into femtosecond with the magnetic pulse

**Figure 13(b)** shows the images of two electron beams with up and down positions at the exit of the compressor, which are called double-decker electron beams. In the experiment, the spot sizes of two laser beams on the photocathode were approximately 1 mm. The distance between the two beams was 2 mm in the vertical direction. The signals of the double-decker electron beams measured by a current transformer are shown in **Figure 13(c)**. The electron charges of double-decker beams were 0.47 nC per pulse (up-beam) and 0.65 nC per pulse (down beam). The different charges were due to the different pulse energies of two laser beams produced in the beam splitter. The time interval of the double-decker electron pulses was 1.4 ns, which is equal to four periods (4 0.35 ns) of the accelerating 2856-MHz rf. The double-decker electron beams have a low emittance of

2.5 0.6 mm-mrad for the up beam and 3.6 0.7 mm-mrad for the down beam, and a low relative energy spread of 0.14 0.03% for both the beams. These enable to compress the double-decker pulses into femtosecond with the magnetic pulse compressor. **Figure 14** shows the temporal distributions of the double-decker electron pulses after the pulse compression measured by a femtosecond streak camera [31]. The pulse duration was obtained to 430 25 fs FWHM for the up beam and 510 20 fs FWHM for the down beam. The difference in the pulse duration for the double pulses was due to the different pulse charge. The two beams

nized electron pulse and analyzing light pulse for pulse radiolysis.

*analyzing laser light was 800 nm. Copyright 2009, with permission from Elsevier.*

**Figure 11.**

*(a) Schematic of electron pulse rotation using the booster linac and magnetic pulse compressor. (b) 2D images of electron pulses with different rotation angle at exit of the pulse compressor measured by a femtosecond streak camera at the accelerating rf phase of ϕrf = 94°, 99°, and 103° [33]. Copyright 2009, with permission from Elsevier.*

electron pulses was 2 ps at φ = 0° and 5 ps at φ = 41°, as shown in **Figure 11(b)**. The double-pulse measurement with time interval of 12.6 ns, output of the 79.3-MHz Ti:sapphire laser oscillator, was used. The duration of the laser pulses was 107 fs RMS. The sizes of the electron beam and the analyzing laser light used in EVS were *σ<sup>l</sup>* = 2 mm and *σ<sup>e</sup>* = 3 mm, respectively.

**Figure 12** gives the transient absorption kinetics of hydrated electrons in water measured by EVS. The rise time of the signals was 11.4 ps with the unrotated electron pulses and was reduced to 6.4 ps with the rotated electron pulses. A higher optical density was obtained with the rotated electron pulses. Under the experimental conditions, we calculated the time resolution due to GVM in the sample to g(L) = 8.7 ps using the unrotated electron pulses (φ = 0°) and g(L) = 0 ps using the rotated electron pulses (φ = 41°), according to Eq. (12). On the other hand, the time resolution due to the electron beam and the analyzing light was calculated in Eq. (7) to be *δtb* ffi 2 ps for the unrotated electron pulses and *δtb* ffi 5 ps for the rotated electron pulses. The total time resolution in EVS pulse radiolysis was estimated to be *Δτ* = 10.7 ps using unrotated electron pulses and *Δτ* = 5 ps using unrotated electron pulses, which are in a good agreement with the obtained rise time in the measurements using both the unrotated and rotated electron pulses.

The results indicate that (1) EVS is a very promising technique to circumvent the degradation of the time resolution due to GVM in the sample, and (2) higher optical densities can be obtained in EVS with the rotated electron pulses because two pulses overlap at every point in the sample and a thick sample can be used. However, the technique of the electron pulse rotation without the increase of the pulse duration remains to be developed.

*Femtosecond Pulse Radiolysis DOI: http://dx.doi.org/10.5772/intechopen.91691*

**Figure 12.**

electron pulses was 2 ps at φ = 0° and 5 ps at φ = 41°, as shown in **Figure 11(b)**. The double-pulse measurement with time interval of 12.6 ns, output of the 79.3-MHz Ti:sapphire laser oscillator, was used. The duration of the laser pulses was 107 fs RMS. The sizes of the electron beam and the analyzing laser light used in EVS were

*(a) Schematic of electron pulse rotation using the booster linac and magnetic pulse compressor. (b) 2D images of electron pulses with different rotation angle at exit of the pulse compressor measured by a femtosecond streak camera at the accelerating rf phase of ϕrf = 94°, 99°, and 103° [33]. Copyright 2009, with permission from Elsevier.*

**Figure 12** gives the transient absorption kinetics of hydrated electrons in water

The results indicate that (1) EVS is a very promising technique to circumvent the degradation of the time resolution due to GVM in the sample, and (2) higher optical densities can be obtained in EVS with the rotated electron pulses because two pulses overlap at every point in the sample and a thick sample can be used. However, the technique of the electron pulse rotation without the increase of the

measured by EVS. The rise time of the signals was 11.4 ps with the unrotated electron pulses and was reduced to 6.4 ps with the rotated electron pulses. A higher optical density was obtained with the rotated electron pulses. Under the experimental conditions, we calculated the time resolution due to GVM in the sample to g(L) = 8.7 ps using the unrotated electron pulses (φ = 0°) and g(L) = 0 ps using the rotated electron pulses (φ = 41°), according to Eq. (12). On the other hand, the time resolution due to the electron beam and the analyzing light was calculated in Eq. (7) to be *δtb* ffi 2 ps for the unrotated electron pulses and *δtb* ffi 5 ps for the rotated electron pulses. The total time resolution in EVS pulse radiolysis was estimated to be *Δτ* = 10.7 ps using unrotated electron pulses and *Δτ* = 5 ps using unrotated electron pulses, which are in a good agreement with the obtained rise time in the measure-

ments using both the unrotated and rotated electron pulses.

*σ<sup>l</sup>* = 2 mm and *σ<sup>e</sup>* = 3 mm, respectively.

**Figure 11.**

*Novel Imaging and Spectroscopy*

**40**

pulse duration remains to be developed.

*Transient absorption kinetics of hydrated electrons in water measured in EVS pulse radiolysis with the rotated electron pulses (blue solid dots) and unrotated electron pulses (red circles) [33]. The wavelength of the analyzing laser light was 800 nm. Copyright 2009, with permission from Elsevier.*

### **3.4 Double-decker electron beam accelerator**

Recently, progress of the advanced particle accelerator and ultrashort laser technologies enables the generation of attosecond electron and laser pulses. When using such ultrashort electron and laser pulses in pulse radiolysis, the time jitter between the two pulses becomes a serious problem. Here, we introduce a technology of double-decker electron beam accelerator to generate both entirely synchronized electron pulse and analyzing light pulse for pulse radiolysis.

**Figure 13** shows the concept of the double-decker electron beam accelerator for pulse radiolysis [31, 32]. The output of Nd:YLF picosecond laser was divided by a beam splitter to produce 2-ps laser beams with different positions in the vertical direction. The 2-ps laser beams were illuminated the photocathode in the rf gun. An optical delay was mounted in the up beam line to time delay the up beam. Two electron beams generated in the rf gun were accelerated in the booster linac up to 32 MeV and were then compressed into femtosecond with the magnetic pulse compressor.

**Figure 13(b)** shows the images of two electron beams with up and down positions at the exit of the compressor, which are called double-decker electron beams. In the experiment, the spot sizes of two laser beams on the photocathode were approximately 1 mm. The distance between the two beams was 2 mm in the vertical direction. The signals of the double-decker electron beams measured by a current transformer are shown in **Figure 13(c)**. The electron charges of double-decker beams were 0.47 nC per pulse (up-beam) and 0.65 nC per pulse (down beam). The different charges were due to the different pulse energies of two laser beams produced in the beam splitter. The time interval of the double-decker electron pulses was 1.4 ns, which is equal to four periods (4 0.35 ns) of the accelerating 2856-MHz rf. The double-decker electron beams have a low emittance of 2.5 0.6 mm-mrad for the up beam and 3.6 0.7 mm-mrad for the down beam, and a low relative energy spread of 0.14 0.03% for both the beams. These enable to compress the double-decker pulses into femtosecond with the magnetic pulse compressor. **Figure 14** shows the temporal distributions of the double-decker electron pulses after the pulse compression measured by a femtosecond streak camera [31]. The pulse duration was obtained to 430 25 fs FWHM for the up beam and 510 20 fs FWHM for the down beam. The difference in the pulse duration for the double pulses was due to the different pulse charge. The two beams

In pulse radiolysis, the front electron pulse was converted to the femtosecond analyzing light with Cherenkov radiation, while the back electron pulse irradiated the sample. A band-pass filter was used to select a necessary wavelength for measurement. An optical delay was used to change the time delay between the Cheren-

kov light pulse and the irradiating electron pulse. In the demonstration, we succeeded to observe the time evolution and spectrum of transient absorption of

In this chapter, we introduced a femtosecond pulse radiolysis instrument, including the generation of femtosecond electron pulses, synchronization of femtosecond analyzing light with the electron pulses, a technique of double-pulse measurement to improve S/N ratio, and the first observation of femtosecond formation processes of hydrated electrons in water. A time resolution of 240 fs due to the electron beam and the analyzing laser has been achieved [26–29]. The femtosecond pulse radiolysis enables the observation of the transient absorption in a thin sample cell with an optical length of 0.18 mm, and paves the way to observe the short-lived intermediate species and primary processes in radiation chemistry and biology on

Two innovative techniques namely "equivalent velocity spectroscopy (EVS)" and "double-decker electron beam accelerator" were presented for next pulse radiolysis development. EVS enables to circumvent entirety the effect of group velocity mismatch (GVM) between the electron beam and the analyzing light in the sample. The double-decker electron beam technique can be expected to reduce the time jitter between the electron pulse and the light pulse to attosecond. Moreover, the double-decker electron beam accelerator enables to generate both ultrashort pulse electron beam and analyzing light. Of course, the electron pulse rotation without the increase of pulse duration and some other problems remain to be solved; however, a combination of the EVS technology with the double-decker electron beams enables the development of ultrafast pulse radiolysis with sub-femtosecond or attosecond time resolution to reveal the entire primary processes beginning ioniza-

hydrated electrons in water [32].

*DOI: http://dx.doi.org/10.5772/intechopen.91691*

*Femtosecond Pulse Radiolysis*

**4. Conclusion**

femtosecond timescale.

**Author details**

**43**

tion in radiation chemistry and biology.

provided the original work is properly cited.

Jinfeng Yang\*, Koichi Kan, Masao Gohdo and Yoichi Yoshida

\*Address all correspondence to: yang@sanken.osaka-u.ac.jp

The Institute of Scientific and Industrial Research, Osaka University, Osaka, Japan

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

#### **Figure 13.**

*(a) Top view and side view of double-decker electron beam accelerator, (b) images and (c) signals of double-decker electron beams [31]. Copyright 2009, with permission from American Institute of Physics.*

#### **Figure 14.**

*Temporal distributions of the double-decker electron pulses after the pulse compression measured by a femtosecond streak camera [31]. The pulse duration of the up beam was 430 25 fs FWHM (green solid line), while the pulse duration of the down beam was 510 20 fs FWHM (blue dashed line). Copyright 2009, with permission from American Institute of Physics.*

were generated by a laser and accelerated within an accelerating rf pulse. The theoretical estimation indicates that the time jitter in double-decker electron pulses can be reduced to attosecond by using a stable accelerating rf.

## *Femtosecond Pulse Radiolysis DOI: http://dx.doi.org/10.5772/intechopen.91691*

In pulse radiolysis, the front electron pulse was converted to the femtosecond analyzing light with Cherenkov radiation, while the back electron pulse irradiated the sample. A band-pass filter was used to select a necessary wavelength for measurement. An optical delay was used to change the time delay between the Cherenkov light pulse and the irradiating electron pulse. In the demonstration, we succeeded to observe the time evolution and spectrum of transient absorption of hydrated electrons in water [32].

## **4. Conclusion**

In this chapter, we introduced a femtosecond pulse radiolysis instrument, including the generation of femtosecond electron pulses, synchronization of femtosecond analyzing light with the electron pulses, a technique of double-pulse measurement to improve S/N ratio, and the first observation of femtosecond formation processes of hydrated electrons in water. A time resolution of 240 fs due to the electron beam and the analyzing laser has been achieved [26–29]. The femtosecond pulse radiolysis enables the observation of the transient absorption in a thin sample cell with an optical length of 0.18 mm, and paves the way to observe the short-lived intermediate species and primary processes in radiation chemistry and biology on femtosecond timescale.

Two innovative techniques namely "equivalent velocity spectroscopy (EVS)" and "double-decker electron beam accelerator" were presented for next pulse radiolysis development. EVS enables to circumvent entirety the effect of group velocity mismatch (GVM) between the electron beam and the analyzing light in the sample. The double-decker electron beam technique can be expected to reduce the time jitter between the electron pulse and the light pulse to attosecond. Moreover, the double-decker electron beam accelerator enables to generate both ultrashort pulse electron beam and analyzing light. Of course, the electron pulse rotation without the increase of pulse duration and some other problems remain to be solved; however, a combination of the EVS technology with the double-decker electron beams enables the development of ultrafast pulse radiolysis with sub-femtosecond or attosecond time resolution to reveal the entire primary processes beginning ionization in radiation chemistry and biology.

## **Author details**

Jinfeng Yang\*, Koichi Kan, Masao Gohdo and Yoichi Yoshida The Institute of Scientific and Industrial Research, Osaka University, Osaka, Japan

\*Address all correspondence to: yang@sanken.osaka-u.ac.jp

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

were generated by a laser and accelerated within an accelerating rf pulse. The theoretical estimation indicates that the time jitter in double-decker electron pulses

*Temporal distributions of the double-decker electron pulses after the pulse compression measured by a femtosecond streak camera [31]. The pulse duration of the up beam was 430 25 fs FWHM (green solid line), while the pulse duration of the down beam was 510 20 fs FWHM (blue dashed line). Copyright 2009, with*

*(a) Top view and side view of double-decker electron beam accelerator, (b) images and (c) signals of double-decker electron beams [31]. Copyright 2009, with permission from American Institute of Physics.*

can be reduced to attosecond by using a stable accelerating rf.

**Figure 13.**

*Novel Imaging and Spectroscopy*

**Figure 14.**

**42**

*permission from American Institute of Physics.*

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[23] Aoki Y, Nkajyo T, Tsunemi A, Yang J, Okada Y, Yorozu M, et al. Performance of compact pulse radiolysis system using a photocathode RF gun. Research on Chemical Intermediates. 2001;**27**:689-697. DOI: 10.1163/

[24] Marignier J-L, Waele V, Monard H, Gobert F, Larbre J-P, Demarque A, et al. Time-resolved spectroscopy at the picosecond laser-triggered electron accelerator ELYSE. Radiation Physics and Chemistry. 2006;**75**:1024-1033. DOI:

10.1016/j.radphyschem.2005.10.020

[25] Kawaguchi M, Ushida K,

Kashiwagi S, Kuroda R, Kuibayashi T, Kobayashi M, et al. Development of compact picosecond pulse radiolysis system. Nuclear Instruments and Methods: Section B. 2005;**236**:425-431. DOI: 10.1016/j.nimb.2005.04.012

[26] Yang J, Kan K, Naruse N, Yoshida Y, Tanimura K, Urakawa J. Femtosecond pulse radiolysis and femtosecond electron diffraction. Nuclear

Instruments and Methods in Physics Research A. 2011;**637**:S24-S29. DOI:

10.1016/j.nima.2010.02.014

[27] Yang J, Kondoh T, Kan K, Yoshida Y. Ultrafast pulse radiolysis. Nuclear Instruments and Methods in Physics Research A. 2010;**629**:6-10. DOI: 10.1016/j.nima.2010.11.109

[28] Yang J, Kondoh T, Kan K, Kozawa T, Yoshida Y, Tagawa S. Femtosecond single electron bunch generation by rotating longitudinal bunch phase space in magnetic field. Nuclear Instruments and Methods in Physics Research A. 2006;**556**:52-56. DOI: 10.1016/j.nima.2005.10.115

15685670152621951

Methods: Section B. 1985;**10**:1004-1006. DOI: 10.1016/0168-583X(85)90158-2

[17] Yoshida Y, Tagawa S, Washio M, Kobayashi H, Tabata Y. Picosecond pulse radiolysis on geminate ion recombination and formation of solute excited state in liquid cyclohexane. Radiation Physics and Chemistry. 1989; **34**:493-496. DOI: 10.1016/1359-0197

[18] Yoshida Y, Ueda T, Kobayashi T, Shibata H, Tagawa S. Studies of geminate ion recombination and formation of excited states in liquid n-dodecane by means of a new picosecond pulse radiolysis system. Nuclear Instruments and Methods A. 1993;**327**:41-43. DOI: 10.1016/0168-9002(93)91405-C

[19] Yoshida Y, Mizutani Y, Kozawa T, Saeki A, Seki S, Tagawa S, et al. Development of laser-synchronized picosecond pulse radiolysis system. Radiation Physics and Chemistry. 2001; **60**:313-318. DOI: 10.1016/S0969-806X

251-253. DOI: 10.1016/S0168-9002(99)

[21] Wishart JF, Cook AR, Miller JR. The LEAF picosecond pulse radiolysis facility at Brookhaven National Laboratory. The Review of Scientific Instruments. 2004;**75**:4359-4365. DOI:

[22] Muroya Y, Lin M, Han Z, Kumagai Y, Sakumi A, Ueda T, et al. Ultra-fast pulse radiolysis: A review of the recent system

(85)90195-5

(89)90051-9

(00)00368-6

00997-3

**45**

10.1063/1.1807004

[20] Kozawa T, Mizutani Y, Miki M, Yamamoto M, Suemine S, Yoshida Y, et al. Development of subpicosecond pulse radiolysis system. Nuclear Instruments and Methods: Section A. 2000;**440**:

[9] Kondoh T, Yang J, Norizawa K, Kan K, Yoshida Y. Femtosecond pulse radiolysis study on geminate ion recombination in n-dodecane. Radiation Physics and Chemistry. 2011;**80**: 286-290. DOI: 10.1016/j. radphyschem.2010.07.049

[10] Kondoh T, Yang J, Norizawa K, Kan K, Kozawa T, Ogata A, et al. Femtosecond pulse radiolysis study of geminate ion recombination in biphenyl–dodecane solution. Radiation Physics and Chemistry. 2013;**84**:30-34. DOI: 10.1016/j.radphyschem.2012. 06.051

[11] Allen AO. The Radiation Chemistry of Water and Aqueous Solutions. Princeton, N.J.: Van Nostrand; 1961

[12] Rodgers MAJ. Farhataziz, Radiation Chemistry: Principles and Applications. VCH: Weinheim; 1987

[13] Hart EJ, Boag JW. Absorption spectrum of the hydrated electron in water and in aqueous solutions. Journal of the American Chemical Society. 1962; **84**:4090-4095. DOI: 10.1021/ ja00880a025

[14] Bronskill MJ, Taylor WB, Wolff RK, Hunt JW. Design and performance of a pulse radiolysis system capable of picosecond time resolution. The Review of Scientific Instruments. 1970;**41**: 333-340. DOI: 10.1063/1.1684511

[15] Tabata Y, Kobayashi H, Washio M, Tagawa S, Yoshida Y. Pulse radiolysis with picosecond time resolution. Radiation Physics and Chemistry. 1985; *Femtosecond Pulse Radiolysis DOI: http://dx.doi.org/10.5772/intechopen.91691*

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[11] Allen AO. The Radiation Chemistry of Water and Aqueous Solutions. Princeton, N.J.: Van Nostrand; 1961

[12] Rodgers MAJ. Farhataziz, Radiation Chemistry: Principles and Applications.

[14] Bronskill MJ, Taylor WB, Wolff RK, Hunt JW. Design and performance of a pulse radiolysis system capable of picosecond time resolution. The Review of Scientific Instruments. 1970;**41**: 333-340. DOI: 10.1063/1.1684511

[15] Tabata Y, Kobayashi H, Washio M, Tagawa S, Yoshida Y. Pulse radiolysis with picosecond time resolution. Radiation Physics and Chemistry. 1985;

[13] Hart EJ, Boag JW. Absorption spectrum of the hydrated electron in water and in aqueous solutions. Journal of the American Chemical Society. 1962;

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VCH: Weinheim; 1987

ja00880a025

286-290. DOI: 10.1016/j. radphyschem.2010.07.049

06.051

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DOI: 10.1063/1.435636

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[5] Tagawa S, Washio M, Kobayashi H, Katsumura Y, Tabata Y. Picosecond pulse radiolysis studies on geminate ion

hydrocarbon. Radiation Physics and Chemistry. 1983;**21**:45-52. DOI: 10.1016/

[6] Tagawa S, Hayashi N, Yoshida Y, Washio M, Tabata Y. Pulse radiolysis studies on liquid alkanes and related polymers. Radiation Physics and Chemistry. 1989;**34**:503-511. DOI: 10.1016/1359-0197(89)90053-2

[7] Yoshida Y, Tagawa S, Kobayashi H, Tabata Y. Study of geminate ion recombination in a solute-solvent system by using picosecond pulse radiolysis. Radiation Physics and Chemistry. 1987;**30**:83-87. DOI: 10.1016/1359-0197(87)90088-9

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[2] Long FH, Lu H, Eisenthal KB. Femtosecond studies of the presolvated electron: An excited state of the solvated electron? Physical Review Letters. 1990;

[16] Kobayashi H, Tabata Y. A 20 ps time resolved pulse radiolysis using two linacs. Nuclear Instruments and Methods: Section B. 1985;**10**:1004-1006. DOI: 10.1016/0168-583X(85)90158-2

[17] Yoshida Y, Tagawa S, Washio M, Kobayashi H, Tabata Y. Picosecond pulse radiolysis on geminate ion recombination and formation of solute excited state in liquid cyclohexane. Radiation Physics and Chemistry. 1989; **34**:493-496. DOI: 10.1016/1359-0197 (89)90051-9

[18] Yoshida Y, Ueda T, Kobayashi T, Shibata H, Tagawa S. Studies of geminate ion recombination and formation of excited states in liquid n-dodecane by means of a new picosecond pulse radiolysis system. Nuclear Instruments and Methods A. 1993;**327**:41-43. DOI: 10.1016/0168-9002(93)91405-C

[19] Yoshida Y, Mizutani Y, Kozawa T, Saeki A, Seki S, Tagawa S, et al. Development of laser-synchronized picosecond pulse radiolysis system. Radiation Physics and Chemistry. 2001; **60**:313-318. DOI: 10.1016/S0969-806X (00)00368-6

[20] Kozawa T, Mizutani Y, Miki M, Yamamoto M, Suemine S, Yoshida Y, et al. Development of subpicosecond pulse radiolysis system. Nuclear Instruments and Methods: Section A. 2000;**440**: 251-253. DOI: 10.1016/S0168-9002(99) 00997-3

[21] Wishart JF, Cook AR, Miller JR. The LEAF picosecond pulse radiolysis facility at Brookhaven National Laboratory. The Review of Scientific Instruments. 2004;**75**:4359-4365. DOI: 10.1063/1.1807004

[22] Muroya Y, Lin M, Han Z, Kumagai Y, Sakumi A, Ueda T, et al. Ultra-fast pulse radiolysis: A review of the recent system

progress and its application to study on initial yields and solvation processes of solvated electrons in various kinds of alcohols. Radiation Physics and Chemistry. 2008;**77**:1176-1182. DOI: 10.1016/S0969-806X(00)00367-4

[23] Aoki Y, Nkajyo T, Tsunemi A, Yang J, Okada Y, Yorozu M, et al. Performance of compact pulse radiolysis system using a photocathode RF gun. Research on Chemical Intermediates. 2001;**27**:689-697. DOI: 10.1163/ 15685670152621951

[24] Marignier J-L, Waele V, Monard H, Gobert F, Larbre J-P, Demarque A, et al. Time-resolved spectroscopy at the picosecond laser-triggered electron accelerator ELYSE. Radiation Physics and Chemistry. 2006;**75**:1024-1033. DOI: 10.1016/j.radphyschem.2005.10.020

[25] Kawaguchi M, Ushida K, Kashiwagi S, Kuroda R, Kuibayashi T, Kobayashi M, et al. Development of compact picosecond pulse radiolysis system. Nuclear Instruments and Methods: Section B. 2005;**236**:425-431. DOI: 10.1016/j.nimb.2005.04.012

[26] Yang J, Kan K, Naruse N, Yoshida Y, Tanimura K, Urakawa J. Femtosecond pulse radiolysis and femtosecond electron diffraction. Nuclear Instruments and Methods in Physics Research A. 2011;**637**:S24-S29. DOI: 10.1016/j.nima.2010.02.014

[27] Yang J, Kondoh T, Kan K, Yoshida Y. Ultrafast pulse radiolysis. Nuclear Instruments and Methods in Physics Research A. 2010;**629**:6-10. DOI: 10.1016/j.nima.2010.11.109

[28] Yang J, Kondoh T, Kan K, Kozawa T, Yoshida Y, Tagawa S. Femtosecond single electron bunch generation by rotating longitudinal bunch phase space in magnetic field. Nuclear Instruments and Methods in Physics Research A. 2006;**556**:52-56. DOI: 10.1016/j.nima.2005.10.115

[29] Yang J, Kondoh T, Kozawa T, Yoshida Y, Tagawa S. Pulse radiolysis based on a femtosecond electron beam and a femtosecond laser light with double-pulse injection technique. Radiation Physics and Chemistry. 2006; **75**:1034-1040. DOI: 10.1016/j. radphyschem.2005.09.016

[30] Oulianov DA, Crowell RA, Gosztola DJ, Shkrob IA, Korovyanko OJ, Rey-de-Castro RC. Ultrafast pulse radiolysis using a terawatt laser Wakefield accelerator. Journal of Applied Physics. 2007;**101**:053102. DOI: 10.1063/1.2696204

[31] Yang J, Kondoh T, Yoshida A, Yoshida Y. Double-decker femtosecond electron beam accelerator for pulse radiolysis. The Review of Scientific Instruments. 2006;**77**:043302. DOI: 10.1063/1.2195090

[32] Kan K, Kondoh T, Yang J, Ogata A, Norizawa K, Yoshida Y. Development of double-decker pulse radiolysis. The Review of Scientific Instruments. 2012; **83**:073302. DOI: 10.1063/1.4731652

[33] Yang J, Kondoh T, Norizawa K, Yoshida Y, Tagawa S. Breaking timeresolution limits in pulse radiolysis. Radiation Physics and Chemistry. 2009; **78**:1164-1168. DOI: 10.1016/j. radphyschem.2009.07.017

[34] Wishart JF, Rao BSM. Recent Trends in Radiation Chemistry. New Jersey: World Scientific Publishing Co. Pte. Ltd.; 2010

[35] Yang J, Kondoh T, Yoshida Y, Tagawa S. Experimental studies of transverse and longitudinal beam dynamics in photoinjector. Japanese Journal of Applied Physics. 2005;**44**: 8702-8707. DOI: 10.1143/JJAP.44.8702

[36] Yang J, Sakai F, Yorozu M, Okada Y, Yanagida T, Endo A. Experimental studies of emittance growth and energy spread in a photocathode RF gun. Nuclear Instruments and Methods in

Physics Research A. 2002;**491**:15-22. DOI: 10.1016/S0168-9002(02)01181-6

**Chapter 4**

**Abstract**

**1. Introduction**

in the IR range.

**47**

*Hiroaki Matsui*

Surface Plasmons and Optical

Dynamics on Vanadium Dioxide

We report on plasmonic resonances on VO2 nanodot arrays and associated optical dynamics. The plasmon excitations based on electric field interactions lead to red shifts of the plasmon resonances to lower photon energy with increasing nanodot size. The spectral linewidths of plasmon peaks gradually become narrow with increasing nanodot size. This is related to a reduction in plasmon damping with respect to the electronic band structure of VO2. This specific band structure of VO2 affects the optical dynamics of plasmon resonances at the sub-picosecond scale. The optical excitations of VO2 comprise intraband and interband transitions. The existence of plasmon bands induces long-lived lifetimes on decay processes. Intraband transitions in the conduction band (C.B.) play an important role in producing long lifetimes, attributing to free carriers in the C.B. By contrast, interband transitions related to bound electrons contribute to plasmon damping. The dynamic optical responses are closely related to the electronic band structures of VO2.

**Keywords:** VO2, surface plasmon, infrared, dynamics, Mott insulator

Recently, plasmonic materials based on oxide and compound semiconductors (e.g., ZnO, CuSe, and InN) have received much attention given that plasmonic responses can be tuned by external fields [1–5]. Investigation of these materials has led to the identification of a new family of plasmonic materials in the infrared (IR) range, which differ from noble metals (e.g., Ag and Au) that have fixed free electron densities. Oxide and compound semiconductors show ideal Drude terms in the IR range due to the absence of interband transitions in the band gap [6]. Plasmonic tuning can be effected by carrier injections due to control of the Fermi level in the electronic bands [7, 8]. The optical features arising from these emerging plasmonic semiconductors show promise for use in optical applications

Control of free carriers has been reported on oxide materials with strong electron-electron correlations. Of these, materials comprising vanadium dioxide (VO2) show a sharp insulator-metal transition (IMT) based on Mott-related and Peierls-related processes [9], which can be controlled by external fields such as thermal, electrical, and optical inputs. In particular, dramatic change of the specific band structure of VO2 with external fields provides more than a three orders of magnitude change in electrical conductance. The IMT character of VO2 resulting

[37] Sakai F, Yang J, Yorozu M, Okada Y, Yanagida T, Endo A. Stable highbrightness electron beam system with a photocathode RF gun for short pulse X-ray generation by Thomson scattering. Japanese Journal of Applied Physics. 2002;**41**:1589-1594. DOI: 10.1143/JJAP.41.1589

[38] Yang J, Sakai F, Yanagida T, Yorozu M, Okada Y, Takasago K, et al. Low-emittance electron-beam generation with laser pulse shaping in photocathode radio-frequency gun. Journal of Applied Physics. 2002;**92**:1608-1612. DOI: 10.1063/ 1.1487457

[39] Nozawa I, Kan K, Yang J, Ogata A, Kondoh T, Gohdo M, et al. Measurement of < 20 fs bunch length using coherent transition radiation. Physical Review Accelerators and Beams. 2014;**17**:072803. DOI: 10.1103/ PhysRevSTAB. 17.072803

[40] Yang J, Yoshida Y. Ultrafast pulse radiolysis for observation of short-lived intermediate species in radiation chemistry. Radioisotopes. 2017;**66**(10): 395-406. DOI: 10.3769/radioisotopes. 66.395

[41] Elles CG, Jailaubekov AE, Crowell RA, Bradforth SE. Excitationenergy dependence of the mechanism for two-photon ionization of liquid H2O and D2O from 8.3 to 12.4eV. The Journal of Chemical Physics. 2006;**125**:044515. DOI: 10.1063/1.2217738

[42] Pimblott SM, Laverne JA, Bartels DM, Jonah CD. Reconciliation of transient absorption and chemically scavenged yields of the hydrated electron in radiolysis. The Journal of Physical Chemistry. 1996;**100**: 9412-9415. DOI: 10.1021/jp960816f

## **Chapter 4**

[29] Yang J, Kondoh T, Kozawa T, Yoshida Y, Tagawa S. Pulse radiolysis based on a femtosecond electron beam and a femtosecond laser light with double-pulse injection technique. Radiation Physics and Chemistry. 2006;

*Novel Imaging and Spectroscopy*

Physics Research A. 2002;**491**:15-22. DOI: 10.1016/S0168-9002(02)01181-6

[37] Sakai F, Yang J, Yorozu M, Okada Y, Yanagida T, Endo A. Stable highbrightness electron beam system with a photocathode RF gun for short pulse X-ray generation by Thomson

scattering. Japanese Journal of Applied Physics. 2002;**41**:1589-1594. DOI:

generation with laser pulse shaping in photocathode radio-frequency gun.

[39] Nozawa I, Kan K, Yang J, Ogata A,

Measurement of < 20 fs bunch length using coherent transition radiation. Physical Review Accelerators and Beams. 2014;**17**:072803. DOI: 10.1103/

[40] Yang J, Yoshida Y. Ultrafast pulse radiolysis for observation of short-lived intermediate species in radiation chemistry. Radioisotopes. 2017;**66**(10): 395-406. DOI: 10.3769/radioisotopes.

Crowell RA, Bradforth SE. Excitationenergy dependence of the mechanism for two-photon ionization of liquid H2O and D2O from 8.3 to 12.4eV. The Journal of Chemical Physics. 2006;**125**:044515.

Bartels DM, Jonah CD. Reconciliation of transient absorption and chemically scavenged yields of the hydrated electron in radiolysis. The Journal of Physical Chemistry. 1996;**100**: 9412-9415. DOI: 10.1021/jp960816f

[38] Yang J, Sakai F, Yanagida T, Yorozu M, Okada Y, Takasago K, et al.

Low-emittance electron-beam

Journal of Applied Physics. 2002;**92**:1608-1612. DOI: 10.1063/

Kondoh T, Gohdo M, et al.

PhysRevSTAB. 17.072803

[41] Elles CG, Jailaubekov AE,

DOI: 10.1063/1.2217738

[42] Pimblott SM, Laverne JA,

1.1487457

66.395

10.1143/JJAP.41.1589

**75**:1034-1040. DOI: 10.1016/j. radphyschem.2005.09.016

[30] Oulianov DA, Crowell RA,

[31] Yang J, Kondoh T, Yoshida A, Yoshida Y. Double-decker femtosecond electron beam accelerator for pulse radiolysis. The Review of Scientific Instruments. 2006;**77**:043302. DOI:

[32] Kan K, Kondoh T, Yang J, Ogata A, Norizawa K, Yoshida Y. Development of double-decker pulse radiolysis. The Review of Scientific Instruments. 2012; **83**:073302. DOI: 10.1063/1.4731652

[33] Yang J, Kondoh T, Norizawa K, Yoshida Y, Tagawa S. Breaking timeresolution limits in pulse radiolysis. Radiation Physics and Chemistry. 2009;

**78**:1164-1168. DOI: 10.1016/j. radphyschem.2009.07.017

Pte. Ltd.; 2010

**46**

[34] Wishart JF, Rao BSM. Recent Trends in Radiation Chemistry. New Jersey: World Scientific Publishing Co.

[35] Yang J, Kondoh T, Yoshida Y, Tagawa S. Experimental studies of transverse and longitudinal beam dynamics in photoinjector. Japanese Journal of Applied Physics. 2005;**44**: 8702-8707. DOI: 10.1143/JJAP.44.8702

[36] Yang J, Sakai F, Yorozu M, Okada Y, Yanagida T, Endo A. Experimental studies of emittance growth and energy spread in a photocathode RF gun. Nuclear Instruments and Methods in

10.1063/1.2696204

10.1063/1.2195090

Gosztola DJ, Shkrob IA, Korovyanko OJ, Rey-de-Castro RC. Ultrafast pulse radiolysis using a terawatt laser Wakefield accelerator. Journal of Applied Physics. 2007;**101**:053102. DOI:
