## 3 Results  
In order to obtain an insight into the ionization dynamics occurring in various parts of the crystal (with respect to the beam focus), below, we show the XTANT predictions for three fluence values, corresponding to 100%, 50%, and 10% of the nominal fluence, which yielded the average absorbed doses of 2.70, 1.35, and 0.27 eV/atom, respectively, within the Si layer down to the photon penetration depth (\(\sim\)30 \(\upmu\)m for 5.95 keV photons). Our simulations predict that the sample evolution proceeds as a sequence of steps, similarly to the graphitization and amorphization of diamond as described in [30; 42]. First, atoms are ionized by energetic X-ray photons. The predominant ionization channel is the excitation of photoelectrons from the K-shells of the Si atoms. The core holes relax predominantly through Auger decay (KLL) on the timescale of 1.5 fs [43], emitting Auger electrons. Figure 1a shows the number of K-shell holes per atom as a function of time. By \(\sim\)30 fs after the maximum of the pump pulse all core holes have decayed. Further, high energy electrons released by X-ray photons and during Auger processes ionize atoms collisionally [33; 40]. The fraction of those non-thermalized electrons is shown in Figure 1b. The subsequent electron cascading saturates by \(\sim\)100 fs. As a result, valence electrons are excited into the conduction band during and for some time after the pulse. Figure 1c shows that, for 100% of the nominal fluence, the number of thermalized electrons in the conduction band reaches 13% already during the exposure to the pump pulse, exceeding the threshold for nonthermal structural damage in silicon (\(\sim\)7-10%) [34]. This damage threshold denotes the minimal fraction of excited electrons (calculated with respect to the initial number of valence electrons per atom), which is needed to initiate a nonthermal structural transition in silicon. For the lower fluence values (50% and 10% of the estimated nominal fluence), the excited electron fraction stayed below 8% and 2%, respectively. In the case of 50% of the nominal fluence (which yields the lower limit of the nonthermal damage threshold), the irradiated crystal subunit will most likely undergo a thermal structural transition where the damage threshold lies for silicon at \(\sim\)5-7% [34]. This transition is much slower than the nonthermal melting, typically occurring on a picosecond timescale. For the case of 10% of the nominal fluence, the fraction of thermalized electrons in the conduction band stayed below both the thermal and nonthermal damage thresholds, and, therefore, neither thermal nor nonthermal structural transition are expected. The energy absorbed during X-ray irradiation is initially transferred only to the electronic system. Thermalized electrons within the valence and the conduction band  
Figure 1: Theoretical predictions for: (**a**) Fraction of K-shell holes per atom, (**b**) fraction of non-thermalized energetic electrons excited per atom, (**c**) fraction of thermalized electrons in the bottom of the conduction band per atom, and (**d**) temperatures of thermalized electrons and ions in silicon crystal after irradiation with an X-ray pulse of 5.95 keV photon energy, duration of 25 fs FWHM, at the absorbed doses of 2.70 eV/atom, 1.35 eV/atom, and 0.27 eV/atom. These correspond to 100%, 50%, and 10% of the nominal peak fluence, respectively. The FEL pump pulse temporal shape is schematically represented by a black solid line. Time zero corresponds to the maximum of the pump pulse. become quickly heated. This is reflected by the fast increase of the electron temperature (Figure 1c) for thermalized electrons in the valence and conduction bands, which occurs on a timescale of electron excitation. Maximal levels of electron excitation are already reached within 50 fs (for 10% of the nominal fluence) up to 100 fs (for 100% of the nominal fluence) since the maximum of the pump pulse. By that time also all K-shell holes have decayed.