**2. FELs as tools for fullerene dynamics**

endohedral fullerenes, ultrafast and ultra-intense free-electron lasers (FELs) are described; we

cific FEL: the Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory. In recent years, the nonlinear physics research in atoms, molecules, and clusters that were conducted using strong laser fields has led to various phenomena, such as the generation of attosecond pulses [5]. The behavior of molecules in short, intense laser fields [6] was extended to large molecules, such as C60, which is intriguing due to the numerous nuclei-electron responses exhibited, because it is a cage of 60 atoms with 240 valence-electrons [7–16]. The interaction of such a large system is key to investigating many-body problems induced on the system's electrons by the photon electric field. The photon interaction with the electronic fullerene's degrees of freedom results in electronic dynamics that lead to nuclei dynamics, because they are both interconnected. Fullerenes, including endohedral fullerenes, are ideal candidates to explore their many-body responses to electromagnetic fields because they respond in different ways—depending upon the field parameters [7–17]. Ionization, which is

The laser photoionization mechanisms of fullerenes have been found to be wavelength and pulse duration-dependent [9, 10, 18]. For IR pulses (800 nm) of about 30 fs duration and inten-

C60, while tunneling, and/or over-the-barrier ionization and ionization due to induced electron re-collision [8] have a low probability to occur under these conditions. The single-activeelectron (SAE) method was used to calculate the ionization of C60 in intense, 4 × 1013 W/cm2 laser pulses with durations between 27 and 70 fs, and for a wide range of wavelengths ranging from 395 to 1800 nm [19]. This calculation agreed with measurements by Shchatsinin et al. [12]. For a long IR wavelength of 1800 nm and 70 fs pulse duration, the SAE picture predicts

tron spectroscopy in addition to the ion measurements allowed new questions to be posed, such as the impact of multi-electron dynamics and whether the ionization and the fragmentation dynamics be adequately modeled in the SAE picture [12]. This latter work resulted in other recent experimental and theoretical investigations, which concluded that both SAE and

Trimetallic nitride template (TNT) endohedral metallofullerenes (EMFs), which consist of a trimetallic nitride moiety and a fullerene host, have also sparked broad interest in many fields—including materials chemistry, organic chemistry, biomedicine, biomedical chemistry, and molecular device design [20–23]. In addition to the fundamental photodynamics interest, EMFs carry the expectation or hope to act as radiotherapy agents to treat tumors while significantly reducing the X-ray dose for patients. Functional groups can be attached to the endohedral fullerene shell to bind the molecules to a specific site in order to deliver toxic, high-Z metal atoms, which are enclosed inside [24, 25]. Endohedral fullerenes have high stability, which is an inherent advantage for resisting biologically induced cage-opening [26].

, it was found that multiphoton processes dominate when ionizing

q+ (q = 1–12) [7]. At a short wavelength of 355 nm, the excitation of

W/cm2

N@C80 fullerene with a spe-

, leading to non-fragmented

emission, as well

[15]. The use of elec-

focus our report on the interaction of C60 and of the endohedral Ho3

50 Fullerenes and Relative Materials - Properties and Applications

one of the possible reactions, has shown to occur on different time scales.

"over the barrier" ionization for a peak intensity of 1015 W/cm2

as other fragments—even for small intensities of about 2 × 10<sup>6</sup>

C60 with 10 ns pulses leads to fragmentation by delayed ionization and C<sup>2</sup>

sities below 5 × 1013 W/cm2

but highly charged C<sup>60</sup>

many-electron effects are important [8].

A new class of intense and short-wavelength lasers, the FELs [27–32], has opened up new research opportunities for many scientific fields, from physics to chemistry, as well as matter under extreme conditions and biology. These vuv/X-ray lasers are accelerator-based tools, which are hybrid, as far as their attributes are concerned, between synchrotron facilities and typical tabletop lasers. FEL typically employs linear accelerators to drive relativistic electron beams through long undulators, characterized by alternating magnetic field, in order to produce intense radiation [32, 33]. **Figure 1** shows a schematic of the LCLS FEL undulator [34]. This type of insertion device, which was key for high-brightness third-generation synchrotron light sources, enable, if they are long enough, the production of ultra-intense vuv/X-ray radiation with femtosecond (fs) pulse duration [27, 30–32].

The use of FELs is unique despite the availability of fs and attosecond tabletop lasers to investigate ultrafast fullerenes and cluster dynamics. FELs add new attributes to tabletop lasers because they provide very high fluence (>10<sup>18</sup> photon/pulse) in a wide, tunable photon energy range (10 eV–12 keV) that is not yet achievable with any tabletop laser. One of the essential attributes of the use of short wavelengths is to enable element selectivity, which permits selecting a specific atom in any system. Furthermore, short wavelengths allow for site selectivity, which

**Figure 1.** Schematic of the 33 m long LCLS FEL undulator [34].

thus targets a specific atomic shell in any system. This provides atomic-scale spatial resolution, which is very important for delineating the effects of each atom in a given system. Because FELs target atomic orbitals instead of molecular orbitals, they allow simple measurements of the response of inner-shell electrons (localized with each atom composing the molecule), compared to the complex response of the molecular orbitals composed of all valence electrons. Probing inner-shell electrons with short wavelengths allows efficient probing of physical and chemical phenomena in fullerenes or clusters from within, in an inside-out ionization.

20–100 μJ, between 14 and 62 eV, with ~100 fs pulse duration, and the spectral width is typically Δ*λ*/*λ* ≈ 1 x 10−3 (FWHM). The combination of these parameters surpasses tabletop lasers. Progress toward the X-ray FEL pulse shape is happening; however, the LCLS has introduced a so-called self-seeding, where a tunable element—typically a crystal or grating monochromator—is inserted halfway through the undulators to filter a specific wavelength for further amplification. This provides higher stability and reduces the laser bandwidth from 20 to 0.5 eV (e.g., 0.75 eV bandwidth at 8450 eV photon energy). The XFEL, in Hamburg, Germany, which will be operating in the fall of 2017, will utilize a new technology based on superconducting linacs, in order to accelerate the electrons that produce the FEL photon pulses, with higher brightness and a higher repetition (rep) rate. This high rep rate of ~1 MHz enables highly differential measurements, such as e-ion-ion coincidence techniques that can delineate dynamics. These coincidence techniques will allow the examination of both electronic and nuclear dynamics, in

Fullerene Dynamics with X-Ray Free-Electron Lasers http://dx.doi.org/10.5772/intechopen.70769 53

real time, subsequent to photo-absorption, which is important for ultrafast chemistry.

of resonance structures and enables wavelength-dependent experiments.

photons occur, leading to sequential multiphoton ionization.

**Figure 2.** Schematic of the photoionization and Auger decay mechanism.

If one were to compare tabletop lasers and FELs, he or she would state that they are complementary light sources, with their own advantages and challenges. The drawback with the tabletop lasers is the need to overcome their low-pulse intensity in the xuv-X-ray regime, as well as their short-range frequency tunability. They can provide weak intensity attosecond pulses, thus enabling electronic dynamics research in some cases [36]. Tabletop lasers deliver higher rep rates (~100 kHz) compared to FELs (120 Hz). However, as stated above, the XFEL in Germany will offer MHz rep rates in 2017. FELs, like synchrotron light sources, interact with inner-shell electrons instead of molecular orbitals. In fact, all FELs produce high-pulse intensity in the X-ray regime, which is ten orders of magnitude higher compared to synchrotron radiation. The broad tunability, with the use of monochromators, enables the exploration

All short-wavelength light sources enable inner-shell ionization, which is followed by the Auger decay—thus, the absorption of one photon leads to the emission of two electrons, as shown in **Figure 2**. With ultra-intense FELs, the fluence is so high that multiple absorptions of

Tabletop laser technology is also progressing to shorten the wavelength and increase the pulse energy, in order to enable study of photon-matter interactions as well—by ionizing or scattering from inner-shell electrons. Short-wavelength FELs also provide femtosecond pulse duration to study nuclear dynamics. Tabletop laser technology has already progressed to provide attosecond pulses, thus allowing the probing of electronic properties of given systems. In parallel, FEL scientists are also working hard at shortening their photon pulses; for example, the LCLS is aiming to produce ~600–400 attoseconds X-ray pulses in the fall of 2017. This new development will allow time-resolved studies of not only nuclear [5] but electronic molecular dynamics as well [35]. These new FEL-based xuv or X-ray photon sources initiate the photodynamics by core ionization. The initial charge is induced with high degrees of temporal and spatial localization, thus making the data analysis easier to handle. The new FEL family of ultrafast xuv/X-ray complement and extend the work carried out by tabletop lasers [36], xuv [37], synchrotron [38], or photon sources.

FELs are emerging photon tools that have been available since 2005, with the first vuv FLASH FEL at DESY in Germany [31] becoming available to scientists. The first X-ray FEL, the LCLS, was commissioned and available to scientists while being commissioned in 2009 [27, 30]. One of the most important attributes of FELs is their tunability within a wide frequency range. They currently span a photon energy range of about 10 eV to 12 keV, with pulse energies that exceed 3 mJ. This includes a repetition (rep) rate of up to 120 Hz, including a fs time scale where the pulse duration can be as short as 2–3fs and as long as 500 fs. The LCLS FEL aims to provide ~600–400 attoseconds in the soft X-ray regime by December 2017, which is an unprecedented progress. This new breakthrough will push the frontiers of science and may result in new science.

In addition to the X-ray LCLS FEL in the United States [27, 30] and the hard X-ray SACLA FEL in Japan's Riken laboratory [28], there are four vuv FEL counterparts: FLASH-1 and FLASH-2 at DESY [31], Germany; SCSS in Spring-8 Japan [28]; and FERMI in Trieste, Italy [29]. Other FELs that will be available this year, in 2017, are as follows: the large European XFEL project at DESY, Germany, will be available in the fall of 2017; a hard and soft X-ray FEL in South Korea called PAL-XFEL; and the SwissFEL at the Paul Scherrer Institute in Switzerland. Other FELS are either amidst planning in Sweden or in construction for the soft-X ray LCLS-II FEL in the United States.

Most FELs produce their photons via the self-amplified spontaneous emission (SASE) process, which is a stochastic process; thus, FEL laser pulses build up from noise and consist of a number of randomly spaced spikes of 1–5 fs duration within an energy envelope of about 15 eV—if it is not monochromatized—giving rise to *not* well-defined laser pulse profiles. The only exception so far is the FERMI FEL, which is a laser-seeded FEL that has similar pulse profiles compared to tabletop lasers. In addition, the FERMI FEL [29] produces a high-pulse energy, 20–100 μJ, between 14 and 62 eV, with ~100 fs pulse duration, and the spectral width is typically Δ*λ*/*λ* ≈ 1 x 10−3 (FWHM). The combination of these parameters surpasses tabletop lasers. Progress toward the X-ray FEL pulse shape is happening; however, the LCLS has introduced a so-called self-seeding, where a tunable element—typically a crystal or grating monochromator—is inserted halfway through the undulators to filter a specific wavelength for further amplification. This provides higher stability and reduces the laser bandwidth from 20 to 0.5 eV (e.g., 0.75 eV bandwidth at 8450 eV photon energy). The XFEL, in Hamburg, Germany, which will be operating in the fall of 2017, will utilize a new technology based on superconducting linacs, in order to accelerate the electrons that produce the FEL photon pulses, with higher brightness and a higher repetition (rep) rate. This high rep rate of ~1 MHz enables highly differential measurements, such as e-ion-ion coincidence techniques that can delineate dynamics. These coincidence techniques will allow the examination of both electronic and nuclear dynamics, in real time, subsequent to photo-absorption, which is important for ultrafast chemistry.

thus targets a specific atomic shell in any system. This provides atomic-scale spatial resolution, which is very important for delineating the effects of each atom in a given system. Because FELs target atomic orbitals instead of molecular orbitals, they allow simple measurements of the response of inner-shell electrons (localized with each atom composing the molecule), compared to the complex response of the molecular orbitals composed of all valence electrons. Probing inner-shell electrons with short wavelengths allows efficient probing of physical and

Tabletop laser technology is also progressing to shorten the wavelength and increase the pulse energy, in order to enable study of photon-matter interactions as well—by ionizing or scattering from inner-shell electrons. Short-wavelength FELs also provide femtosecond pulse duration to study nuclear dynamics. Tabletop laser technology has already progressed to provide attosecond pulses, thus allowing the probing of electronic properties of given systems. In parallel, FEL scientists are also working hard at shortening their photon pulses; for example, the LCLS is aiming to produce ~600–400 attoseconds X-ray pulses in the fall of 2017. This new development will allow time-resolved studies of not only nuclear [5] but electronic molecular dynamics as well [35]. These new FEL-based xuv or X-ray photon sources initiate the photodynamics by core ionization. The initial charge is induced with high degrees of temporal and spatial localization, thus making the data analysis easier to handle. The new FEL family of ultrafast xuv/X-ray complement and extend the work carried out by tabletop lasers [36], xuv

FELs are emerging photon tools that have been available since 2005, with the first vuv FLASH FEL at DESY in Germany [31] becoming available to scientists. The first X-ray FEL, the LCLS, was commissioned and available to scientists while being commissioned in 2009 [27, 30]. One of the most important attributes of FELs is their tunability within a wide frequency range. They currently span a photon energy range of about 10 eV to 12 keV, with pulse energies that exceed 3 mJ. This includes a repetition (rep) rate of up to 120 Hz, including a fs time scale where the pulse duration can be as short as 2–3fs and as long as 500 fs. The LCLS FEL aims to provide ~600–400 attoseconds in the soft X-ray regime by December 2017, which is an unprecedented progress.

This new breakthrough will push the frontiers of science and may result in new science.

In addition to the X-ray LCLS FEL in the United States [27, 30] and the hard X-ray SACLA FEL in Japan's Riken laboratory [28], there are four vuv FEL counterparts: FLASH-1 and FLASH-2 at DESY [31], Germany; SCSS in Spring-8 Japan [28]; and FERMI in Trieste, Italy [29]. Other FELs that will be available this year, in 2017, are as follows: the large European XFEL project at DESY, Germany, will be available in the fall of 2017; a hard and soft X-ray FEL in South Korea called PAL-XFEL; and the SwissFEL at the Paul Scherrer Institute in Switzerland. Other FELS are either amidst planning in Sweden or in construction for the soft-X ray LCLS-II FEL in the United States. Most FELs produce their photons via the self-amplified spontaneous emission (SASE) process, which is a stochastic process; thus, FEL laser pulses build up from noise and consist of a number of randomly spaced spikes of 1–5 fs duration within an energy envelope of about 15 eV—if it is not monochromatized—giving rise to *not* well-defined laser pulse profiles. The only exception so far is the FERMI FEL, which is a laser-seeded FEL that has similar pulse profiles compared to tabletop lasers. In addition, the FERMI FEL [29] produces a high-pulse energy,

chemical phenomena in fullerenes or clusters from within, in an inside-out ionization.

[37], synchrotron [38], or photon sources.

52 Fullerenes and Relative Materials - Properties and Applications

If one were to compare tabletop lasers and FELs, he or she would state that they are complementary light sources, with their own advantages and challenges. The drawback with the tabletop lasers is the need to overcome their low-pulse intensity in the xuv-X-ray regime, as well as their short-range frequency tunability. They can provide weak intensity attosecond pulses, thus enabling electronic dynamics research in some cases [36]. Tabletop lasers deliver higher rep rates (~100 kHz) compared to FELs (120 Hz). However, as stated above, the XFEL in Germany will offer MHz rep rates in 2017. FELs, like synchrotron light sources, interact with inner-shell electrons instead of molecular orbitals. In fact, all FELs produce high-pulse intensity in the X-ray regime, which is ten orders of magnitude higher compared to synchrotron radiation. The broad tunability, with the use of monochromators, enables the exploration of resonance structures and enables wavelength-dependent experiments.

All short-wavelength light sources enable inner-shell ionization, which is followed by the Auger decay—thus, the absorption of one photon leads to the emission of two electrons, as shown in **Figure 2**. With ultra-intense FELs, the fluence is so high that multiple absorptions of photons occur, leading to sequential multiphoton ionization.

**Figure 2.** Schematic of the photoionization and Auger decay mechanism.
