Resonant inelastic x-ray scattering in warm-dense Fe compounds beyond the SASE FEL resolution limit

Abstract

Resonant inelastic x-ray scattering (RIXS) is a widely used spectroscopic technique, providing access to the electronic structure and dynamics of atoms, molecules, and solids. However, RIXS requires a narrow bandwidth x-ray probe to achieve high spectral resolution. The challenges in delivering an energetic monochromated beam from an x-ray free electron laser (XFEL) thus limit its use in few-shot experiments, including for the study of high energy density systems. Here we demonstrate that by correlating the measurements of the self-amplified spontaneous emission (SASE) spectrum of an XFEL with the RIXS signal, using a dynamic kernel deconvolution with a neural surrogate, we can achieve electronic structure resolutions substantially higher than those normally afforded by the bandwidth of the incoming x-ray beam. We further show how this technique allows us to discriminate between the valence structures of Fe and Fe₂O₃, and provides access to temperature measurements as well as M-shell binding energies estimates in warm-dense Fe compounds.

The authors combine differentiable physics modelling and neural networks to extract high-resolution electronic density of states of warm dense materials from resonant inelastic x-ray scattering spectra. With this approach, they identify distinctive features in the valence structures of warm dense Fe and Fe2O3, also estimating their temperature and M-shell binding energies.