Theory and Simulation of Transient Absorption in Warm Dense Matter Created by an X-Ray Free-Electron Laser*

The understanding of  warm dense matter (WDM) is important in many fields, including planetary science, inertial confinement fusion, and  astrophysics. Although recent advances in experimental techniques using x-ray free electron lasers have led to probes of WDM at extreme temperature- and time-scales, theoretical interpretations of these out of equilibrium states of matter are only now being developed. Here we describe an approach for theoretical simulations of x-ray absorption in XFEL generated warm dense copper based on the combination of a kinetic Boltzmann equation model and  finite-temperature real-space Green's function theory. Calculations of the L₃ near edge spectra reproduce the main features and trends of recent experimental data [1], including a transition from reverse-saturable to saturable absorption over a wide range of ultra-short fs duration x-ray pulse intensities. The interpretation of these results links   features in the spectra to a combination of core- and valence-level shifts, band narrowing, and changes in the photoelectron inelastic mean free path that result from depletion of the 3d band  with increased pulse intensities.