Modeling Femtosecond Reduction of Atomic Scattering Factors in X-ray-Excited Silicon with Boltzmann Kinetic Equations

In this communication, we describe the application of Boltzmann kinetic equations for modeling massive electronic excitation in a silicon nanocrystal film after its irradiation with intense femtosecond hard X-ray pulses. This analysis was inspired by an experiment recently performed at the X-ray free-electron laser facility SACLA, which measured a significant reduction in atomic scattering factors triggered by an X-ray pulse of the intensity ∼1019 W/cm2, occurring on a timescale comparable with the X-ray pulse duration (6 fs full width at half maximum). We show that a Boltzmann kinetic equation solver can accurately follow the details of the electronic excitation in silicon atoms caused by such a hard X-ray pulse, yielding predictions in very good agreement with the experimental data.