Transient Absorption of Warm Dense Matter Created by an X-Ray Free-Electron Laser

Abstract

Warm dense matter (WDM) is a ubiquitous state of matter encountered in various high energy density environments, at the frontier between a plasma and a condensed phase. It is key to astrophysics, planetary science and inertial confinement fusion research, but its electronic and ionic structure and dynamics remain poorly understood. Here, an intense and ultrafast X-ray free electron laser (XFEL) pulse is used to simultaneously create and characterize warm dense copper via L-edge X-ray absorption spectroscopy. The rich electron dynamics occurring within the 15-femtosecond pulse duration are revealed over a large irradiation intensity range. Below 1015 W cm-2, an absorption peak below the L-edge appears, originating from the depletion of the 3d band, and shows a distinct energy redshift with increasing intensity. This shift is a possible signature of ionization potential depression and electronic bond hardening. At higher intensities, massive ionization and collisions lead to the transition from reverse saturable absorption (opening absorption channels) to saturable absorption (material bleaching) of the XFEL pulse, an effect that holds promise for X-ray pulse shaping. The presented method is applicable to a wide range of materials and constitutes a robust benchmark for non-equilibrium models of electron dynamics in WDM.

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

“…This scheme, indeed, significantly restricts the number of active atomic configurations during a simulation with the Boltzmann model. In this way, computationally efficient simulations of atomic excitation and relaxation in heavier materials, such as gold [38] and copper [39], have become feasible.…”

Application of Boltzmann kinetic equations to model X-ray-created warm dense matter and plasma

“…It is key to astrophysics, planetary science and inertial confinement fusion research, but its electronic and ionic structure and dynamics remain poorly understood. Among the methods used to diagnose WDM, spectroscopic methods play a very important role [13][14][15][16][17].…”

“…Also, the Boltzmann tool can accurately follow non-equilibrium stages of sample evolution, including complicated atomic excitation and relaxation paths also in heavy materials, which is important for spectroscopic applications, e.g. [ 17 ].…”

“…It limited the number of active atomic configurations by following the sample excitation and relaxation only along the most probable relaxation paths (including predominant collisional processes). That scheme enabled computationally efficient simulations of the complicated pathways of atomic excitation and relaxation in heavy materials, such as gold [ 45 ] and copper [ 17 ].…”