Relaxation pathways in x-ray free-electron-laser heated iron

Nonthermal photoionized plasmas are now established in the laboratory and require models that treat the atomic processes and electron distribution self-consistently. We investigate the effects of inelastic thermalization in iron under intense x-ray irradiation using the atomic model BigBarT, suited for the self-consistent evolution of the electron continuum, including degeneracy effects. Our study focuses particularly on collisional M-shell ionization, which we identify as the dominant relaxation process of the nonthermal electrons. We show that M-shell satellite intensities are sensitive to nonthermal ionization, providing a potential method to refine collisional cross sections that are otherwise difficult to compute due to their proximity to the continuum and the associated plasma screening effects.