**3. Ionization and fragmentation dynamics of C60 with the LCLS and FEL**

The ionization and fragmentation of C60 were investigated experimentally and theoretically by using intense, fs, X-ray FEL pulses. The goal was to understand the response of a relatively large, strongly bonded molecule and to quantitatively understand the fs dynamical effects initiated by intense X-ray exposure. This is important to fundamental nonlinear physics research progress in general. It also directly impacts both the study of matter under extreme conditions and the area of biomolecular imaging. Imaging viruses and proteins, at an atomic spatial scale and on the time scale of atomic motion, requires quantitative understanding—which is best accomplished through spectroscopic measurements of a system that is simpler than a biomolecule—instead of scattering experiments. In other words, studying fullerenes, this is the first step toward understanding in a large system how a multiply ionized fullerene leads to slow electrons, which in turn initiate radiation damage in biological systems [39]. Therefore, the photoionization of C60 with intense FEL pulse is relevant to imaging of biomolecules, because such a study revealed the influence of processes that were not known or reported prior to our work [40].

The structure determination of biomolecules is done by imaging these systems using X-ray scattering experiments. These studies need photon brightness to obtain the images, which unfortunately induces electronic and structural radiation damage—thus altering the sample despite the use of short pulse durations [41]. Calculations have shown that highintensity X-ray pulses trigger a cascade of damage processes in ferredoxin crystals, which are a metalloprotein of particular interest to the biology community [41, 42]. Furthermore, intense X-ray FEL pulses were found to modify the electronic properties of C60 on a fs time scale, based on observations of the diffraction of intense 32-fs X-ray pulses by a powder sample of crystalline C60 [41]. These radiation damages need to be evaluated, and spectroscopic measurements can reveal information that is not available with scattering experiments.

C60 was chosen as a benchmark molecule because of its chemically bonded carbon atoms, whose bond lengths and damage processes can emulate biomolecules. The fs experimental and theoretical C60 investigation dynamics was studied in the gas phase, with intense 485 eV photon energy to take advantage of the large photo-absorption cross section of carbon 1s electrons. We aimed to reach conditions in which approximately each C atom within a C60 molecule in the focus of X-ray pulse absorbs multiple photons during the X-ray pulse duration.

The experiment was carried out at the AMO hutch at the LCLS. The fullerenes (C60 and Ho3 N@ C80) were introduced into the vacuum through a heated oven source. For the C60 experiment, a 2m-long-magnetic-bottle time-of-flight spectrometer [43] was used to measure the ions and electrons that resulted from the ionization [40]. The spectrometer is shown in **Figure 3**, while a zoom of the interaction region is displayed in **Figure 4**; the oven appears as a cylinder from the top, the spectrometer's permanent magnet sticks to the left of the figure, and the spectrometer lens is shown to the right side of **Figure 4**.

We chose three pulse durations 4, 30, and 90 fs to explore the effect of the X-ray pulse duration on the C60 ionization. As explained with **Figure 2**, the inner-shell ionization will lead to the Auger process, but with a high-fluence X-ray FEL pulse, this results in many photo- and Auger electrons due to the cyclic, multiphoton ionization until the pulse duration is over. Additionally, this photoionization process leads to secondary ionization of C60, as well as of its

**Figure 4.** Close-up view of the interaction region showing the nozzle part of the oven, the permanent magnet, and the

**Figure 3.** Picture of the AMO hutch installation. The 2m-long-magnetic-bottle spectrometer, wrapped in aluminum foil,

Fullerene Dynamics with X-Ray Free-Electron Lasers http://dx.doi.org/10.5772/intechopen.70769 55

is shown sticking out from the interaction chamber.

spectrometer lens (see details in the text).

**3. Ionization and fragmentation dynamics of C60 with the LCLS and FEL**

The ionization and fragmentation of C60 were investigated experimentally and theoretically by using intense, fs, X-ray FEL pulses. The goal was to understand the response of a relatively large, strongly bonded molecule and to quantitatively understand the fs dynamical effects initiated by intense X-ray exposure. This is important to fundamental nonlinear physics research progress in general. It also directly impacts both the study of matter under extreme conditions and the area of biomolecular imaging. Imaging viruses and proteins, at an atomic spatial scale and on the time scale of atomic motion, requires quantitative understanding—which is best accomplished through spectroscopic measurements of a system that is simpler than a biomolecule—instead of scattering experiments. In other words, studying fullerenes, this is the first step toward understanding in a large system how a multiply ionized fullerene leads to slow electrons, which in turn initiate radiation damage in biological systems [39]. Therefore, the photoionization of C60 with intense FEL pulse is relevant to imaging of biomolecules, because such a study revealed the influence of processes that were

The structure determination of biomolecules is done by imaging these systems using X-ray scattering experiments. These studies need photon brightness to obtain the images, which unfortunately induces electronic and structural radiation damage—thus altering the sample despite the use of short pulse durations [41]. Calculations have shown that highintensity X-ray pulses trigger a cascade of damage processes in ferredoxin crystals, which are a metalloprotein of particular interest to the biology community [41, 42]. Furthermore, intense X-ray FEL pulses were found to modify the electronic properties of C60 on a fs time scale, based on observations of the diffraction of intense 32-fs X-ray pulses by a powder sample of crystalline C60 [41]. These radiation damages need to be evaluated, and spectroscopic measurements can reveal information that is not available with scattering

C60 was chosen as a benchmark molecule because of its chemically bonded carbon atoms, whose bond lengths and damage processes can emulate biomolecules. The fs experimental and theoretical C60 investigation dynamics was studied in the gas phase, with intense 485 eV photon energy to take advantage of the large photo-absorption cross section of carbon 1s electrons. We aimed to reach conditions in which approximately each C atom within a C60 molecule in the focus of X-ray pulse absorbs multiple photons during the X-ray pulse

The experiment was carried out at the AMO hutch at the LCLS. The fullerenes (C60 and Ho3

C80) were introduced into the vacuum through a heated oven source. For the C60 experiment, a 2m-long-magnetic-bottle time-of-flight spectrometer [43] was used to measure the ions and electrons that resulted from the ionization [40]. The spectrometer is shown in **Figure 3**, while a zoom of the interaction region is displayed in **Figure 4**; the oven appears as a cylinder from the top, the spectrometer's permanent magnet sticks to the left of the figure, and the spectrom-

We chose three pulse durations 4, 30, and 90 fs to explore the effect of the X-ray pulse duration on the C60 ionization. As explained with **Figure 2**, the inner-shell ionization will lead to

N@

not known or reported prior to our work [40].

54 Fullerenes and Relative Materials - Properties and Applications

eter lens is shown to the right side of **Figure 4**.

experiments.

duration.

**Figure 3.** Picture of the AMO hutch installation. The 2m-long-magnetic-bottle spectrometer, wrapped in aluminum foil, is shown sticking out from the interaction chamber.

**Figure 4.** Close-up view of the interaction region showing the nozzle part of the oven, the permanent magnet, and the spectrometer lens (see details in the text).

the Auger process, but with a high-fluence X-ray FEL pulse, this results in many photo- and Auger electrons due to the cyclic, multiphoton ionization until the pulse duration is over. Additionally, this photoionization process leads to secondary ionization of C60, as well as of its fragment ions by the photo- and Auger electrons. The C60 molecule charges up to C60 8+ based on our data. The X-ray pulse is not homogeneous, as described in Section 2. Therefore, depending upon where C60 lands in the X-ray pulse, it can fragment in different scenarios. If C60 lands outside of the focus of the X-ray FEL pulse, it will fragment, as shown in **Figure 5**, into molecular ion strands with either a ring or linear structure. **Figure 5** shows the mass-to-charge ratio (M/Q), displaying the parent C60 ionized up to q = 8+ along with the molecular ions C<sup>m</sup> + , m = 1–11.

If C60 lands in the focus of the FEL pulse, because of the short C─C bond lengths, it fragments via the Coulomb repulsion into small molecular carbon ions and atomic carbon charge-state distribution from C+ to C6+. Therefore, C can be fully stripped with 90 fs pulse duration [40] as shown in **Figure 6**. In this case, the pulse is intense and long enough for the C atom to undergo at least three cyclic photoionization-Auger decays, leading to C6+.

As shown in **Figure 6**, the charge-state distribution depends upon the pulse duration. At the shorter pulse duration we used (4fs), the highest charge state obtained is C5+ indicating that at least two cyclic photoionization-Auger decays occurred [40, 44]. Our measurements (light color data) were compared with calculation (dark color data) in order to understand the different physical and chemical effects, giving rise to the observed charge-state distribution.

**Figure 5.** Time-of-flight spectrum showing the fragmentation of C60 with mid-fluence LCLS X-ray pulses. Light color data are experiment while dark color data are theory.

**Figure 6.** Atomic carbon charge-state distribution resulting from the Coulomb explosion of C60 under high-fluence X-ray FEL pulses. (a) corresponds to data taken with 4f pulse duration while (c) corresponds to data taken with 90 fs pulse duration.

Fullerene Dynamics with X-Ray Free-Electron Lasers http://dx.doi.org/10.5772/intechopen.70769 57

fragment ions by the photo- and Auger electrons. The C60 molecule charges up to C60

1–11.

to C6+.

distribution.

charge-state distribution from C+

56 Fullerenes and Relative Materials - Properties and Applications

data are experiment while dark color data are theory.

our data. The X-ray pulse is not homogeneous, as described in Section 2. Therefore, depending upon where C60 lands in the X-ray pulse, it can fragment in different scenarios. If C60 lands outside of the focus of the X-ray FEL pulse, it will fragment, as shown in **Figure 5**, into molecular ion strands with either a ring or linear structure. **Figure 5** shows the mass-to-charge ratio (M/Q), displaying the parent C60 ionized up to q = 8+ along with the molecular ions C<sup>m</sup>

If C60 lands in the focus of the FEL pulse, because of the short C─C bond lengths, it fragments via the Coulomb repulsion into small molecular carbon ions and atomic carbon

duration [40] as shown in **Figure 6**. In this case, the pulse is intense and long enough for the C atom to undergo at least three cyclic photoionization-Auger decays, leading

As shown in **Figure 6**, the charge-state distribution depends upon the pulse duration. At the shorter pulse duration we used (4fs), the highest charge state obtained is C5+ indicating that at least two cyclic photoionization-Auger decays occurred [40, 44]. Our measurements (light color data) were compared with calculation (dark color data) in order to understand the different physical and chemical effects, giving rise to the observed charge-state

**Figure 5.** Time-of-flight spectrum showing the fragmentation of C60 with mid-fluence LCLS X-ray pulses. Light color

to C6+. Therefore, C can be fully stripped with 90 fs pulse

8+ based on

+ , m =

**Figure 6.** Atomic carbon charge-state distribution resulting from the Coulomb explosion of C60 under high-fluence X-ray FEL pulses. (a) corresponds to data taken with 4f pulse duration while (c) corresponds to data taken with 90 fs pulse duration.

#### **3.1. Discussion**

With current computer technology, it is not possible to carry out ab initio calculations via quantum mechanical methods. The best that can be done is to develop classical physics models that describe the dynamics of atoms/ions and electrons that appear in the continuum after photoionization. These models include, for example, the rate equations of the ionization and the C ionization cross sections that are directly resulting from quantum mechanical calculations [45, 46]. The model used in this FEL-based C60 work consists of a mixed molecular dynamics (MD) Monte Carlo tool based on treating C atoms [47]. The real-space dynamics of atoms, ions, and the (quasi-) free electrons resulting from photoionization and Auger decay is described by Newtonian mechanics. In this methodology, C60 is modeled as 60 individual carbon atoms. The atoms' electronic configuration is tracked and changed stochastically during each time step (0.8 attosecond) by using a Monte Carlo scheme. During the many ionization events, new electrons that are released from their bound atomic orbitals are treated classically, with the appropriate energies, as they "appear" in the system. The C atoms were held together by a fullerenespecific classical Brenner force field, and the charges interacted via Coulomb forces. In order to mimic a molecule, the model needed to contain several physical and chemical processes that were revealed through the experiment. **Figure 6** shows a good agreement between the model and the experiment, but in the initial comparison, they did not agree. The model originally predicted more abundant C ion charge states, which revealed that there was *strong recombination* between the released electrons from photoionization and the ions after the FEL pulse ends.

The comparison between the model and the experimental data not only led to the interpretation of our experiment but also led to solid improvement in the MD calculation. Specifically, the model had to include several physical and chemical processes: (1) bond-breaking in C<sup>60</sup> was modeled by setting the bonding force field to zero between two C-charged ions; (2) the secondary ionizations arising from free electrons colliding with and ionizing atoms/ions was included; (3) the molecular Auger effect consisted of sharing the energy and using two carbon atoms in the release of the two electrons. This was taken into account by removing the ejected electron from the L-shell of a neighboring C atom, while the 1s vacancy of an ion was filled up with its own L-shell electron; and (4) the recombination of a classically trapped electron if it were no longer delocalized among several C ions but instead localized to only one C ion.

one at a time to display their importance. As can be seen, molecular effects and molecular bonds slightly contributed to a better agreement. The dramatic effect is obtained in the case of the secondary ionization of C60 by the photo- and Auger-electrons. This effect is weak in small, isolated molecules and van der Waals clusters; it is completely absent in atoms. As can be seen, the addition of molecular bond-breaking and molecular Auger improved even more the model. This combined theoretical and experimental work illustrates the successful use of *classical mechanics* to describe all moving particles in C60. The work clearly revealed the influence of processes not previously suspected or reported. The impact we aimed to achieve was also realized because this fullerene spectroscopic work quantitatively demonstrated electronic damage due to photoelectron and Auger electrons interacting with the ions as a secondary ionization effect. Our results were corroborated with recent separate calculations [39]. Finally, one of the goals of the MD model was to build a new approach that would scale with larger systems, such as biomolecules. This work demonstrated that the modeling [40, 44] coached by experiment was successful and can be applicable for X-ray interactions with any extended system—even at higher X-ray dose rates that are expected with future FEL sources, such as

Fullerene Dynamics with X-Ray Free-Electron Lasers http://dx.doi.org/10.5772/intechopen.70769 59

the soon-to-be available XFEL in Germany and with the future LCLS-II.

**Figure 7.** Comparison between the experimental data and the model (see text for details).

**Figure 7** shows the effect of the MD modeling, where we compare the data and the model for two pulse durations, 30 and 90 fs. As can be seen, there is no effect with the two different pulse durations, which thus revealed that the dynamics might already be over with the use of a 30 fs X-ray pulse. This means that the physical and chemical processes included might have occurred within 30 fs.

In order to explain the comparison between the model and the data, we display in **Figure 7** the C ion yield *difference* between the model and the experiment on the Y-axis. The smallest value of the bars corresponds to the best agreement between the model and the experiment. Each yield corresponds to the sum of the charge-state distribution (C1+–C6+) shown in **Figure 7**. The X-axis displays bars that correspond to the several different physical and chemical processes, which were included in the model. As described above, each of these processes were included

**Figure 7.** Comparison between the experimental data and the model (see text for details).

**3.1. Discussion**

58 Fullerenes and Relative Materials - Properties and Applications

occurred within 30 fs.

With current computer technology, it is not possible to carry out ab initio calculations via quantum mechanical methods. The best that can be done is to develop classical physics models that describe the dynamics of atoms/ions and electrons that appear in the continuum after photoionization. These models include, for example, the rate equations of the ionization and the C ionization cross sections that are directly resulting from quantum mechanical calculations [45, 46]. The model used in this FEL-based C60 work consists of a mixed molecular dynamics (MD) Monte Carlo tool based on treating C atoms [47]. The real-space dynamics of atoms, ions, and the (quasi-) free electrons resulting from photoionization and Auger decay is described by Newtonian mechanics. In this methodology, C60 is modeled as 60 individual carbon atoms. The atoms' electronic configuration is tracked and changed stochastically during each time step (0.8 attosecond) by using a Monte Carlo scheme. During the many ionization events, new electrons that are released from their bound atomic orbitals are treated classically, with the appropriate energies, as they "appear" in the system. The C atoms were held together by a fullerenespecific classical Brenner force field, and the charges interacted via Coulomb forces. In order to mimic a molecule, the model needed to contain several physical and chemical processes that were revealed through the experiment. **Figure 6** shows a good agreement between the model and the experiment, but in the initial comparison, they did not agree. The model originally predicted more abundant C ion charge states, which revealed that there was *strong recombination* between the released electrons from photoionization and the ions after the FEL pulse ends. The comparison between the model and the experimental data not only led to the interpretation of our experiment but also led to solid improvement in the MD calculation. Specifically, the model had to include several physical and chemical processes: (1) bond-breaking in C<sup>60</sup> was modeled by setting the bonding force field to zero between two C-charged ions; (2) the secondary ionizations arising from free electrons colliding with and ionizing atoms/ions was included; (3) the molecular Auger effect consisted of sharing the energy and using two carbon atoms in the release of the two electrons. This was taken into account by removing the ejected electron from the L-shell of a neighboring C atom, while the 1s vacancy of an ion was filled up with its own L-shell electron; and (4) the recombination of a classically trapped electron if it were no longer delocalized among several C ions but instead localized to only one C ion. **Figure 7** shows the effect of the MD modeling, where we compare the data and the model for two pulse durations, 30 and 90 fs. As can be seen, there is no effect with the two different pulse durations, which thus revealed that the dynamics might already be over with the use of a 30 fs X-ray pulse. This means that the physical and chemical processes included might have

In order to explain the comparison between the model and the data, we display in **Figure 7** the C ion yield *difference* between the model and the experiment on the Y-axis. The smallest value of the bars corresponds to the best agreement between the model and the experiment. Each yield corresponds to the sum of the charge-state distribution (C1+–C6+) shown in **Figure 7**. The X-axis displays bars that correspond to the several different physical and chemical processes, which were included in the model. As described above, each of these processes were included one at a time to display their importance. As can be seen, molecular effects and molecular bonds slightly contributed to a better agreement. The dramatic effect is obtained in the case of the secondary ionization of C60 by the photo- and Auger-electrons. This effect is weak in small, isolated molecules and van der Waals clusters; it is completely absent in atoms. As can be seen, the addition of molecular bond-breaking and molecular Auger improved even more the model.

This combined theoretical and experimental work illustrates the successful use of *classical mechanics* to describe all moving particles in C60. The work clearly revealed the influence of processes not previously suspected or reported. The impact we aimed to achieve was also realized because this fullerene spectroscopic work quantitatively demonstrated electronic damage due to photoelectron and Auger electrons interacting with the ions as a secondary ionization effect. Our results were corroborated with recent separate calculations [39]. Finally, one of the goals of the MD model was to build a new approach that would scale with larger systems, such as biomolecules. This work demonstrated that the modeling [40, 44] coached by experiment was successful and can be applicable for X-ray interactions with any extended system—even at higher X-ray dose rates that are expected with future FEL sources, such as the soon-to-be available XFEL in Germany and with the future LCLS-II.

#### **4. Ionization and fragmentation dynamics of Ho3 N@C80**

The C60 work described above led us to consider exploring increased complexity by choosing the interaction of endohedral fullerenes (also called doped fullerenes) with FELs. These systems are even more intriguing than C60, because they host a moiety that ranges from an atom to a molecule. Nothing was known about their structure or dynamics when excited with X-ray FEL. As mentioned in the introduction, these nanoscale systems have received attention in part because they can be used for applications ranging from medical usage [48] to drug delivery, as well as their possible use for quantum computing [49]. Our interest, however, stems from exploring a fundamental point of view, the fragmentation of Ho3 N@C80 induced by short, X-ray pulses from the LCLS FEL.

The experiment was carried out at the AMO hutch using a time-of-flight spectrometer [50] for detecting the ions produced in the interaction of the endohedral fullerenes with the LCLS pulses. The experiment on Ho<sup>3</sup> N@C80 was carried out with 1530 eV in order to selectively target the ionization from a specific shell, which was the Ho 3d. The pulse duration of the X-ray pulse was 80 fs with a pulse energy of about 6.7 × 1015 photons/cm2 . This experiment had less fluence than the experiment on C60 by two orders of magnitude due to a different transport of the photon beam through the optics [50]. We estimate that the fluence used in the Ho<sup>3</sup> N@C80 is only ¼ of the fluence used in C60 [40], thus resulting in only a few multiphoton ionization cycles. **Figure 8** shows three time-of-flight spectra that result from the multiphoton ionization processes of Ho3 N@C80 with intense X-ray FEL. We show in **Figure 8** three panels that focus on the singly, doubly, and triply ionized parent.

The top panel, which spans the mass-to-charge ratio (M/Q) from 350 to 1300, depicts structures attributed to multiply charged fullerenes' parent ions from singly charged Ho<sup>3</sup> N@C80 + to quintuply charged Ho<sup>3</sup> N@C80 5+ (albeit weak). This clearly shows that the fluence for this experiment is weaker than for the C60 experiment we described above since in **Figure 5** the parent ions were charged up to C60 8+. It is also clear that the atomic Ho+ ion has the highest yield compared to all other ion fragments. This must arise from the fragmentation of the encapsulated Ho3 N moiety—thus, indicating that the three multiply charged Ho atoms, which were selected to be the most ionized compared to N or C atoms, freed themselves from the C<sup>80</sup> cage. The middle panel focuses on the doubly and triply ionized parent molecule along with doubly ionized fullerene molecules that lost C dimers such as Ho<sup>3</sup> N@C78 2+, Ho3 N@C70 2+, and Ho3 N@C50 2+. The loss of C dimers was also observed previously with tabletop experiments [2]; however, the shrunk cage size to C50 was never previously observed. The bottom panel shows the triply and quadruply charged parent fullerenes, along with triply ionized parent fullerenes that lost C dimers, namely, Ho3 N@C78 3+, Ho3 N@C76 3+, Ho3 N@C70 3+, and Ho3 N@C50 3+.

**4.1. Discussion**

eight photons were absorbed by Ho<sup>3</sup>

and reaching at least Ho3

was about 0.013 Mb. Our interpretation of the interaction of Ho<sup>3</sup>

N@C80

This experiment was carried out in the low-fluence regime, since we estimated that about

**Figure 8.** Ion yield M/Q spectra shown in three panels. The top panel depicts a wide M/Q fragment ions, while the

middle and bottom fragments focus on doubly and triply charged parent ions (see text for details).

we appraised that about 180 photons were absorbed per C60 molecule. The photoionization with 1530 eV leads to the absorption cross section of Ho to be about 1.55 Mb, while that of C

energy was that the Ho atom charges up and gets multi-ionized due to the cyclic photoionization and Auger decay [51]. In doing so, it grabs electrons from the carbon cage via electron transfer between the Ho and the cage—resulting in the whole parent molecule charging up

cage charges up, it would become unstable and will break apart, thus leading to molecular

N@C80. In the C60 experiment, described in Section 3,

Fullerene Dynamics with X-Ray Free-Electron Lasers http://dx.doi.org/10.5772/intechopen.70769 61

5+, as observed in **Figure 8**. We assumed that as the carbon

N@C80 with 1530 eV photon

**Figure 9** shows other fragments that occur in the 160–240 M/Q range. The prominent signal is the atomic Ho<sup>+</sup> ion which escaped the cage. Other fragment ions are C molecular ion chains or rings such as C15 + and C17 + along with surprising new fragments such as HoC2 + , HoCN<sup>+</sup> , HoC4 + , and HoC3 N<sup>+</sup> [51].

**Figure 8.** Ion yield M/Q spectra shown in three panels. The top panel depicts a wide M/Q fragment ions, while the middle and bottom fragments focus on doubly and triply charged parent ions (see text for details).

#### **4.1. Discussion**

**4. Ionization and fragmentation dynamics of Ho3**

by short, X-ray pulses from the LCLS FEL.

60 Fullerenes and Relative Materials - Properties and Applications

on the singly, doubly, and triply ionized parent.

N@C80

doubly ionized fullerene molecules that lost C dimers such as Ho<sup>3</sup>

pulses. The experiment on Ho<sup>3</sup>

processes of Ho3

encapsulated Ho3

N@C50

the atomic Ho<sup>+</sup>

or rings such as C15

, and HoC3

Ho3

HoC4 +

to quintuply charged Ho<sup>3</sup>

parent ions were charged up to C60

fullerenes that lost C dimers, namely, Ho3

+ and C17 +

N<sup>+</sup> [51].

The C60 work described above led us to consider exploring increased complexity by choosing the interaction of endohedral fullerenes (also called doped fullerenes) with FELs. These systems are even more intriguing than C60, because they host a moiety that ranges from an atom to a molecule. Nothing was known about their structure or dynamics when excited with X-ray FEL. As mentioned in the introduction, these nanoscale systems have received attention in part because they can be used for applications ranging from medical usage [48] to drug delivery, as well as their possible use for quantum computing [49]. Our interest, however,

The experiment was carried out at the AMO hutch using a time-of-flight spectrometer [50] for detecting the ions produced in the interaction of the endohedral fullerenes with the LCLS

get the ionization from a specific shell, which was the Ho 3d. The pulse duration of the X-ray

fluence than the experiment on C60 by two orders of magnitude due to a different transport of the photon beam through the optics [50]. We estimate that the fluence used in the Ho<sup>3</sup>

is only ¼ of the fluence used in C60 [40], thus resulting in only a few multiphoton ionization cycles. **Figure 8** shows three time-of-flight spectra that result from the multiphoton ionization

The top panel, which spans the mass-to-charge ratio (M/Q) from 350 to 1300, depicts struc-

experiment is weaker than for the C60 experiment we described above since in **Figure 5** the

yield compared to all other ion fragments. This must arise from the fragmentation of the

were selected to be the most ionized compared to N or C atoms, freed themselves from the C<sup>80</sup> cage. The middle panel focuses on the doubly and triply ionized parent molecule along with

[2]; however, the shrunk cage size to C50 was never previously observed. The bottom panel shows the triply and quadruply charged parent fullerenes, along with triply ionized parent

**Figure 9** shows other fragments that occur in the 160–240 M/Q range. The prominent signal is

N@C78

tures attributed to multiply charged fullerenes' parent ions from singly charged Ho<sup>3</sup>

N@C80 with intense X-ray FEL. We show in **Figure 8** three panels that focus

8+. It is also clear that the atomic Ho+

N moiety—thus, indicating that the three multiply charged Ho atoms, which

2+. The loss of C dimers was also observed previously with tabletop experiments

3+, Ho3

ion which escaped the cage. Other fragment ions are C molecular ion chains

along with surprising new fragments such as HoC2

N@C76

3+, Ho3

stems from exploring a fundamental point of view, the fragmentation of Ho3

pulse was 80 fs with a pulse energy of about 6.7 × 1015 photons/cm2

**N@C80**

N@C80 was carried out with 1530 eV in order to selectively tar-

5+ (albeit weak). This clearly shows that the fluence for this

N@C78

N@C70

2+, Ho3

3+, and Ho3

N@C80 induced

N@C80

N@C80 +

2+, and

,

N@C50 3+.

ion has the highest

N@C70

+ , HoCN<sup>+</sup>

. This experiment had less

This experiment was carried out in the low-fluence regime, since we estimated that about eight photons were absorbed by Ho<sup>3</sup> N@C80. In the C60 experiment, described in Section 3, we appraised that about 180 photons were absorbed per C60 molecule. The photoionization with 1530 eV leads to the absorption cross section of Ho to be about 1.55 Mb, while that of C was about 0.013 Mb. Our interpretation of the interaction of Ho<sup>3</sup> N@C80 with 1530 eV photon energy was that the Ho atom charges up and gets multi-ionized due to the cyclic photoionization and Auger decay [51]. In doing so, it grabs electrons from the carbon cage via electron transfer between the Ho and the cage—resulting in the whole parent molecule charging up and reaching at least Ho3 N@C80 5+, as observed in **Figure 8**. We assumed that as the carbon cage charges up, it would become unstable and will break apart, thus leading to molecular

Pump-probe techniques with FELs offer similar opportunities to measure physical and chemical changes in molecules at an atomic spatial resolution (on the time scale of atomic motion). With attosecond, the goal is to also measure the electronic motion. These techniques can be used for the study of fullerenes, in order to tackle fundamental questions such as, *how do the atoms in the fullerenes move after the photon energy is deposited in the fullerenes?* How do the bonds between the atoms that make the fullerenes break, or what are the pathways for the induced atomic motion in fullerenes? These questions can be asked and answered with FELs using pump-probe techniques. As in standard pump-probe work, to capture the dynamics, the pump pulse initiates the motions, and a probe pulse detects the changes using as many time delays as needed between the pump and probe pulses, ideally in a wide time scale. In order to achieve this goal, one needs two pulses, which can be generated in a few ways: (1) the accelerator scientists have developed several methods, but the most recent one cuts fresh slices from the electron bunch. They manipulate the electron bunch before it enters a split undulator, so that only its tail is lasing, in order to produce the first X-ray pulse. Next, they delay the electron bunch in order to acquire a time delay and spoil the electron bunch orbit further, by having the head of the bunch lasing and thus using a fresh slice from the electron bunch to produce the second photon pulse [52]. This scheme is also capable of providing two X-ray colors [52]; (2) the FEL pulse can be split into two X-ray pulses with an X-ray split and delay tool [53]. These two schemes were used successfully in experiments in the mode of X-ray pump-X-ray probe; and (3) a third scheme utilizes an X-ray pulse from the FEL as either the pump or the probe, and it is paired with a short tabletop pulse laser [54] (IR or UV). All of

Fullerene Dynamics with X-Ray Free-Electron Lasers http://dx.doi.org/10.5772/intechopen.70769 63

The FEL-based experiments, paired most of the time with pump-probe techniques, are carried out by using various types of spectrometers or imaging detectors for absorption experiments or for diffractive scattering experiments, respectively. The latter holds the promise to achieve imaging of molecules in the gas phase. Double VMI spectrometers paired with X-ray/IR, for a pump-probe scheme, are also used to measure the electrons—as well as the ions using the ionion coincidence techniques. These differential techniques examine both electronic and nuclear dynamics following the interaction of fullerenes with FEL X-ray pulses and also resolve the transient electronic rearrangement that accompanies photoionization [55]. The current repetition (rep) rate for the operational FEL facilities is at most 120 Hz (lower than the current tabletop repetition rate of 1–100 KHz), which does not allow easy FEL-based electron-ion-ion coincidence studies—although some work has been done in this highly differential mode [56]. However, the new XFEL in Hamburg, Germany, which will be completed by the end of this year, will offer ~1 MHz rep rate—thus allowing the measurement of e-ion-ion coincidences to track and precisely determine the decay channels and time it takes for the ultrafast nuclear dynamics to occur. It seems that the dream of making molecular movies is reachable in the next few years.

The investigation of the ionization and fragmentation of fullerenes with FELs is at its infancy.

N@C80 using ion

This work reported on the first two spectroscopic experiments on C60 and Ho3

these schemes have been used in various FELs.

**6. Conclusions**

**Figure 9.** Ion spectrum displaying M/Q focusing on Ho ion and the Ho-based molecular fragments ion (see text for details).

fragment ions. The Ho atoms are about ten times heavier than the C atoms, and, therefore, we assume that they will not move faster than the carbon cage. We clearly have evidence of bondbreaking and bond-forming since we observed in **Figure 9** the following fragments: HoC<sup>2</sup> + , HoCN<sup>+</sup> , HoC4 + , and HoC3 N<sup>+</sup> . It is unclear if the moiety first breaks into three Ho atoms and the N atoms and then the carbon cage fragments or if the reverse occurs. It is also unclear how the new bonds have formed. This is to be determined by future time-resolved experiments that might track the ionization and fragmentation dynamics and decipher the mechanisms leading to the final ionic states we observed. Additionally, we hope that our work will stimulate the development of molecular dynamics simulations suitable for endohedral fullerenes and for even larger molecules exposed to intense XFEL.
