**1. Introduction**

The behavior of C60 in relation to ionizing radiation is important because it sheds light on the fundamental many-body problem due to the numerous nuclei–electron responses exhibited in a large molecule. Investigating C60 using different light sources may reveal responses, which may impact other research fields. We thus studied C60 using short wavelength lasers, and in particular, with free electron lasers (FELs), which are still relatively new light sources compared to table-top lasers or synchrotron facilities. FELs have opened up new research opportunities because they deliver photons in a new energy regime for many scientific fields, from physics to chemistry, as well as to matter under extreme conditions and biology [1]. These VUV/X-ray lasers are accelerator-based tools, which are a hybrid between synchrotron radiation facilities and typical table-top lasers. FELs produce high brightness radiation with typical femtosecond (fs) pulse duration [2] and have been available since 2005, with the first VUV FLASH FEL at DESY in Germany [3]. There are currently several X-ray FELs around the world in addition to XUV FELs. The first X-ray FEL, the Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory [1,2], was commissioned and made available to scientists in 2009. It was used to carry out the static and time-resolved work reported here. The LCLS so far has a repetition rate up to 120 Hz, including a fs time scale where the pulse duration can be as short as 2–3 fs and as long as 500 fs [1,2]. Since 2017, the LCLS FEL also provides pulses as short as ~280 attoseconds (*as*) in the soft X-ray regime, which is an unprecedented technical progress that already impacts current X-ray *as* research [4].

X-ray FELs have opened up the possibility to investigate the ultra-fast response of matter to intense femtosecond X-ray pulses as well as to their pulse duration. The initial research on atoms and small molecules uncovered new aspects of this response, such as rapid sequences of inner-shell photoionization and the Auger–Meitner decay [5,6]. More recently, fullerenes were investigated with FELs because they bridge the gap between molecules and nanoparticles and are model systems for studying the dynamical behavior of large systems when exposed to intense, X-ray short pulses. Fullerenes have displayed molecular [7] and bulk [8] behavior and have proven to be an excellent testing ground for experiments and theories [9]. The behavior of C60 in relation to ionizing radiation is intriguing due to the numerous nuclei–electron responses exhibited, since it consists

**Citation:** Berrah, N. Probing C60 Fullerenes from within Using Free Electron Lasers. *Atoms* **2022**, *10*, 75. https://doi.org/10.3390/ atoms10030075

Academic Editors: Anatoli Kheifets, Gleb Gribakin and Vadim Ivanov

Received: 20 May 2022 Accepted: 13 July 2022 Published: 14 July 2022

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**Copyright:** © 2022 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

of a cage of 60 atoms with 240 valence electrons [10]. 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, which leads to nuclei dynamics, since they are inter-connected. Thus, the investigation of the interaction of C60 with photons has been carried out extensively, theoretically and experimentally [11], to advance, in some cases, the quantitative understanding of the electronic and nuclear structure of these large molecules.

The photon used by the scientific community to study C60 varied from IR to X-rays, and some studies were static, while others were time dependent, in order to understand the ultra-fast dynamics that arises in these systems subsequent to photoabsorption. The time-resolved experiments and calculation of the interaction of C60 with photons have the ultimate goal to control ultra-fast molecular dynamics and understand the chemical transformation at the fs timescale. These studies include probing the multi-electron interactions in fullerenes as well as between the electrons and the carbon nuclei [12].

Non-linear physics and strong-field table-top laser research with C60 was extensively studied, and the photoionization mechanisms were found to be wavelength and pulse duration dependent [13]. The single-active-electron (SAE) method was used to calculate the ionization of C60 in intense, 4 × 10<sup>13</sup> W/cm<sup>2</sup> laser pulses with durations between 27 and 70 fs and for a wide range of wavelengths ranging from 395 to 1800 nm [14], which agreed with the measurements. For a long I.R. wavelength of 1800 nm and 70 fs pulse duration, the SAE picture predicts "over the barrier" ionization for a peak intensity of 10<sup>15</sup> W/cm2, leading to non-fragmented parent molecule but highly charged C60q+ (q = 1–12) [12] ions. At a short wavelength of 355 nm, the excitation of C60 with 10 ns pulses led to fragmentation by delayed ionization and C2 emission as well as other fragments, even for small intensities of about 2 × 10<sup>6</sup> W/cm<sup>2</sup> [15].

We investigated and we report here on two studies regarding the X-ray ionization and fragmentation of C60 under high- and mid-fluence X-ray femtosecond pulses from the LCLS. One study is static, and the other one is time resolved. The results of the static study demonstrated that intense X-ray FEL multiply ionizes the parent molecules before breaking into molecular ions as well as into highly charged atomic C states. This work contributed toward understanding the radiation damage, and in particular, electronic damage, due to X-ray radiation, which is essential to understand for the progress of bio-molecular imaging. The result of the time-resolved study gave new insights into the dynamics of the C60 fragmentation subsequent to mid-fluence absorption of the X-ray photon. It demonstrated the importance of chemical effects and charge transfer in stabilizing the molecule against fragmentation over several hundred femtoseconds after the X-ray pump pulse. We conclude this article by identifying and providing recommendations for future research opportunities using *as* pump-probe techniques.

#### **2. Materials and Methods**

The measurement of the ions resulting from the ionization and fragmentation dynamics of C60 under X-ray exposure was achieved by using a magnetic bottle spectrometer [16] at the LCLS AMO hutch. The method is described elsewhere [16]; thus, our description here is brief. The C60 sample was produced via a collimated molecular beam using an evaporative oven introducing the gas phase C60 into the vacuum chamber. The oven was resistively heated and had a small nozzle and skimmer through which the C60 molecules entered the interaction region. The oven was heated to ~700–800 K, and a liquid-nitrogen-cooled dump opposite the skimmer captured the target after it passed through the interaction region. X-ray optics focused the incoming X-ray pulses to a peak focal intensity of ~ 10<sup>15</sup> W cm–2. The magnetic bottle spectrometer consisted of a 2 m long ion drift path, providing high ion mass-to-charge and KE resolution. The pulse energies of the two beams, centroid photon energy and pulse duration were recorded and used as statistical filters in the analysis. The interaction region, defined by the intersection of the focused X-ray beam and the molecular

beam, covered a volume around the focus with an inhomogeneous X-ray spatial fluence distribution. Therefore, the measured data contain contributions from a wide range of fluence (volume integrated signal), with peak X-ray fluence only at the center of the focus. To model the interaction region, the X-ray spatial fluence distribution was calibrated using the ion yields from Ar. This spatial fluence distribution was then applied in the modeling of the X-ray interaction with C60 to account for low- and high-fluence regions, allowing quantitative comparison of theoretical predictions to the experimental data.
