Modeling electron dynamics in silicon driven by high-intensity femtosecond x-rays.

High-intensity femtosecond-duration x-rays from free electron lasers have enabled innovative imaging techniques that employ smaller crystal sizes than conventional crystallography. Developments aimed at increasing x-ray pulse intensities bring opportunities and constraints due to ultra-fast changes to atomic scattering form factors from electron dynamics. Experiments on silicon by Inoue [Inoue , Phys. Rev. Lett. , 163201 (2023)] illustrate this by measuring diffraction efficiencies with increasing x-ray pulse intensities. Results at the highest experimental x-ray pulse intensity have been theoretically studied [Inoue , Phys. Rev. Lett. , 163201 (2023); Ziaja , Atoms , 154 (2023)] but not fully reproduced, which raises questions about the mechanisms behind these changes. Using collisional radiative simulations and relativistic configuration-averaged atomic data, we compute the ionization dynamics and diffraction efficiency of silicon and find good agreement within the experimental uncertainty. We incorporate the effects of ionization potential depression by removing energy levels close to the ionization threshold over selected charge states. We identify the main electron impact mechanisms present in our simulations. We bridge the gap between high and low intensity and find regimes where electronic damage affects the efficiency of high- and low-momentum transfer. We computationally examine the effects of free electron degeneracy and find that it does not influence ionization dynamics. Finally, we consider how a non-thermal electron distribution may modify our results. This investigation gives insight into the mechanisms and helps guide future experiments that utilize intense x-ray pulses to achieve high-resolution structural determination.