**4. Discussion**

Our experimental work in the high- [22] and mid-fluence [25] X-ray regime, investigating the response of C60 to fs FEL pulses, revealed new physical and chemical processes that were validated either qualitatively or quantitatively by state-of-the-art simulation methodology. Specifically, the static X-ray high-fluence, 90 fs study showed that the Coulomb explosion subsequent to multiphoton ionization is so violent that it leads to fully stripped C atoms, and the model does not require the inclusion of detailed molecular effects. The most important effect that needed to be included in the calculation was the secondary electron collision with the ions fragments as well as the recapture of electrons by the ion fragments. On the other hand, the time-resolved mid-fluence study demonstrated that chemical bonds and the charge transfer effect were crucial to be included in the calculation to observe dynamics as a function of time delay. These effects had to be included to agree with the measurements that depicted dynamics as a function of time delay. In this situation, the simulations and the experiment revealed that despite significant ionization induced by the ultra-short (20 fs) X-ray pump pulse, the fragmentation of C60 was significantly delayed. This work uncovered the persistence of the molecular structure of C60, which hinders fragmentation over a timescale of hundreds of femtoseconds. Furthermore, the calculation demonstrated that a substantial fraction of the ejected fragments is neutral carbon atoms. In fact, it is these neutral fragments' ejection from C60, as the molecule cools

off, that prohibits a strong Coulomb explosion. These findings, interpreted by the most advanced modeling and theory, provide insights into X-ray FEL-induced radiation damage in large molecules, including biomolecules [25]. In fact, this work seems to indicate that experiments conducted with up to 20 fs, with mid fluence, will not experience radiation damage due to the delayed ionization. Thus, bio-molecule X-ray diffraction at this fluence will not suffer the "diffract before destroy" scenario. Furthermore, our simulation [25] provides a solid basis for the reliable interpretation of processes in systems even larger than C60. Future XFEL-based experimental research into a wide range of systems will benefit from our results and the theoretical advances needed to interpret the experimental work.

Recently, a time-resolved experiment was conducted with intense XUV photons at the FLASH-II FEL that probed the resonance structure at about 20 eV, and this work might reveal new insight regarding the ionization and fragmentation of C60 at lower photon energy [26]. In addition, an experiment was conducted with hard X-rays at LCLS to visualize the light-induced reshaping of C60 via X-ray diffraction. Specifically, time-resolved X-ray diffraction images of C60 molecules were recorded during and after their interaction with intense near-infrared fields, giving direct access to structural changes of the molecules and their neutral or ionic fragmentation, in real time [27].

The current experiments so far focused on measuring the charged fragments resulting from absorption of X-ray photons by neutral C60 to investigate the ionization processes or using X-ray diffraction to visualize the transformation of C60. Future work could investigate the anion, C60− [28,29] or C60+ [30]. Additionally, what is missing are experiments that will use photoelectron spectroscopy to measure in detail the ionization and fragmentation dynamics. In particular, the electron-ion coincidence techniques could be used, especially with the now available high repetition rate at FELs sources, to measure the momenta of the ions as well as identify the possible coincidences among the fragments and to visualize the different coincidence channels. Furthermore, the use of ultra-fast electron diffraction (UED) techniques [31] will allow the imaging of the molecular changes as a function of time delay between the pump and probe pulse.

Finally, the exciting advent of *as* X-ray pulses from FELs opens up the opportunity to study the electronic dynamics that precedes the nuclear dynamics [32]. Specifically, innershell X-ray pump-probe studies with *as* pulses could allow the probing and understanding of the electronic dynamics on its natural timescale. Currently, *as* pulses from the LCLS-II X-ray FEL are provided to scientists with photon energies tunable across the soft X-ray regime and with spectral brightness six orders of magnitude greater than HHG sources [32]. This *as* capability will soon be reproduced at other FEL facilities around the world, enabling previously impossible as experiments in all research fields.
