**1. Introduction**

Fullerenes, which bridge the gap between molecules and nanoparticles, are ideal systems for investigating the dynamical behavior of extended systems when exposed to X-rays. Fullerenes and their derivatives, characterized by their hollow geometric structures and nanometersized outer diameter, draw a great deal of interest due to their wide range of applications and "supramolecular" physical and chemical properties [1]. Fullerenes have displayed molecular [2] and bulk [3] behavior and have proven to be an excellent testing ground for experiments and theories [4].

This chapter, which is not a review, focuses on the interaction of intense lasers with fullerenes—and in particular, with X-ray free-electron lasers (FELs). After brief, general introductions on the interaction of fullerenes with strong laser fields using tabletop lasers and on

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endohedral fullerenes, ultrafast and ultra-intense free-electron lasers (FELs) are described; we focus our report on the interaction of C60 and of the endohedral Ho3 N@C80 fullerene with a specific FEL: the Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory.

This is thought to help protect healthy tissues from the toxic metals. Monochromatic X-rays could be used to resonantly excite the high-Z atom, restricting the X-ray radiation damage to the cells that need to be treated, while leaving surrounding tissues largely unaffected by the radiation dose. In our case, we are interested in exploring the X-ray absorption and response

Fullerene Dynamics with X-Ray Free-Electron Lasers http://dx.doi.org/10.5772/intechopen.70769 51

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 radia-

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

of EMF with ultrashort X-rays.

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

tion with femtosecond (fs) pulse duration [27, 30–32].

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

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

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 intensities below 5 × 1013 W/cm2 , it was found that multiphoton processes dominate when ionizing 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 "over the barrier" ionization for a peak intensity of 1015 W/cm2 , leading to non-fragmented but highly charged C<sup>60</sup> q+ (q = 1–12) [7]. At a short wavelength of 355 nm, the excitation of C60 with 10 ns pulses leads to fragmentation by delayed ionization and C<sup>2</sup> emission, as well as other fragments—even for small intensities of about 2 × 10<sup>6</sup> W/cm2 [15]. The use of electron 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 many-electron effects are important [8].

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]. This is thought to help protect healthy tissues from the toxic metals. Monochromatic X-rays could be used to resonantly excite the high-Z atom, restricting the X-ray radiation damage to the cells that need to be treated, while leaving surrounding tissues largely unaffected by the radiation dose. In our case, we are interested in exploring the X-ray absorption and response of EMF with ultrashort X-rays.
