**6. Conclusions**

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>

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

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

**5. Measuring time-resolved dynamics using pump-probe techniques** 

troscopy seem reachable in the near future due to the new technologies.

Pump-probe spectroscopy techniques, championed by tabletop lasers [36], allow the measurement of dynamics for any system**—**from atoms to fullerenes and from solids to biological specimens. They are being used extensively, and the ultimate goal is to determine the motions and locations of nuclei and electrons and to determine the energy flow and charge transfer in systems. Recording these motions and making "molecular movies" using pump-probe spec-

. It is unclear if the moiety first breaks into three Ho atoms and the

HoCN<sup>+</sup>

details).

**with FELs**

, HoC4 +

, and HoC3

62 Fullerenes and Relative Materials - Properties and Applications

N<sup>+</sup>

for even larger molecules exposed to intense XFEL.

+ ,

> The investigation of the ionization and fragmentation of fullerenes with FELs is at its infancy. This work reported on the first two spectroscopic experiments on C60 and Ho3 N@C80 using ion

spectroscopy but much more needs to be accomplished. These first experiments need to be followed by time-resolved studies to delineate the nuclear dynamics, including electron transfer, using e-ion-ion coincidence techniques or using diffraction techniques and imaging. In addition, the next generation of attosecond FELs will allow the exploration and hopefully understanding of electron migration between atoms using photoelectron spectroscopy. Finally, the work done on C60 was conclusive because of the close interaction with theories and modeling. We hope that the reported work will stimulate theorists to tackle the many-body interactions resulting from the interaction of fullerenes with femtosecond or attosecond X-ray or vuv FELs.

[6] Zhao SF, Le AT, Jin C, Wang X, Lin CD. Analytical model for calibrating laser intensity in strong-field-ionization experiments. Physical Review A. 2016;**93**:023413. DOI: 10.1103/

Fullerene Dynamics with X-Ray Free-Electron Lasers http://dx.doi.org/10.5772/intechopen.70769 65

[7] Bhardwaj VR, Corkum PB, Rayner DM. Internal laser-induced dipole force at work in C60 molecule. Physical Review Letters. 2003;**91**:203004 and references therein. DOI:

[8] Huismans Y, et al. Macro-atom versus many-electron effects in ultrafast ionization of C60.

[9] Campbell EE, et al. From above threshold ionization to statistical electron emission: The laser pulse-duration dependence of C60 photoelectron spectra. Physical Review Letters.

[10] Kjellberg M, et al. Momentum-map-imaging photoelectron spectroscopy of fullerenes with femtosecond laser pulses. Physical Review A. 2010;**81**:023202. DOI: 10.1103/

[11] Bohl E, et al. Relative photoionization cross sections of super-atom molecular orbitals (SAMOs) in C60. The Journal of Physical Chemistry A. 2015;**119**:11504-11508. DOI:

[12] Shchatsinin I, et al. C60 in intense short pulse laser fields down to 9 fs: excitation on time scales below e-e and e-phonon coupling. The Journal of Chemical Physics. 2006;

[13] Campbell EEB, Hoffmann K, Rottke H, Hertel IV. Sequential ionization of C60 with femtosecond laser pulses. The Journal of Chemical Physics. 2001;**114**:1716. DOI: http://

[14] Tchaplyguine MHK et al*.* Ionization and fragmentation of C60 with sub-50 fs laser pulses. The Journal of Chemical Physics. 2000;**112**:2781. DOI: http://dx.doi.org/10.1063/1.480852

[16] Li H, et al. Coherent electronic wave packet motion in C60 controlled by the waveform and polarization of few-cycle laser fields. Physical Review Letters. 2015;**114**:123004. DOI:

[17] Xiong H, Mignolet B, Fang L, Osipov T, Thomas T, Wolf JA, Sistrunk E, Gühr M, Remacle R, Berrah N. The role of super-atom molecular orbitals in doped fullerenes in a femtosec-

[18] Li H, et al. Transition from SAMO to Rydberg state ionization in C60 in femtosecond laser fields. The Journal of Physical Chemistry Letters. 2016;**7**:4677-4682. DOI: 10.1021/

and statistical modeling of kinetic energy release. The Journal of Chemical Physics.

emission: Experiment

**125**:194320. DOI: http://dx.doi.org/10.1063/1.2362817

[15] Lebeault MA, et al. Decay of C60 by delayed ionization and C<sup>2</sup>

2012;**137**:054312. DOI: http://dx.doi.org/10.1063/1.4737926

ond intense laser field. Scientific Reports. 2017;**7**:121

Physical Review A. 2013;**88**:013201. DOI: 10.1103/PhysRevA.88.013201

PhysRevA.93.023413

2000;**84**:2128

PhysRevA.81.061602

10.1021/jp408147f

dx.doi.org/10.1063/1.1584671

10.1103/PhysRevLett.114.123004

acs.jpca.5b11713

10.1103/PhysRevLett.91.203004
