X-ray spectroscopy in non-equilibrium and extreme conditions John Rehr

There has been considerable recent interest in studies of electronic structure and spectra of materials in non-equilibrium and extreme conditions. These include, for example, the warm dense matter (WDM) regime with temperatures of order the Fermi temperature TF. Such studies are of particular interest at next-generation high intensity and pulsed XFEL light sources, where these conditions can be simulated. Complimentary theoretical simulations are essential to interpret the experimental results. Although several approaches have been introduced for calculations of WDM in equilibrium [1], they are usually not directly applicable to transient sources or to spectra. Thus, the development of theoretical methods for treating such exotic conditions remains challenging. In this presentation, we discuss several developments to this end. We first discuss a finite-temperature (FT) cumulant Green’s function approach which yields both thermodynamic properties and FT exchange-correlation potentials vxc(T,n), consistent with the PIMC fits [3]. Next, we address extensions of our real-space Green’s function x-ray absorption spectroscopy (XAS) codes to finite temperature, up to the WDM regime [4]. Finally, we discuss recent theoretical simulations of non-equilibrium, transient, high intensity XAS from XFEL sources. This theory accounts for the transient, ultrafast rearrangement of electronic states and occupations with increasing beam intensity, that explain the observed cross-over from reverse saturable absorption (RSA) to saturable absorption (SA) [5], where the material becomes transparent. *Supported by the Theory Institute for Materials and Energy Spectroscopies (TIMES) at SLAC, FWP 100291, funded by US DOE Contract DE-AC02-76SF0051, with computer support from the National Energy Research Scientific Computer (NERSC).