Theoretical Treatments of the Bound-Free Contribution and Experimental Best Practice in X-ray Thomson Scattering from Warm Dense Matter

TL;DR

This paper compares theoretical models for bound-free contributions in X-ray Thomson scattering from warm dense matter, identifies errors in common approximations, and proposes improved practices for more accurate experimental analysis.

Contribution

It highlights a systematic error in the plane-wave form factor approximation and offers an improved method for analyzing XRTS data in warm dense matter research.

Findings

Identified energy conservation violation in PWFFA approximation.

Demonstrated improved accuracy with proposed bound-free treatment.

Raised questions about the reliability of state variable inference from low-resolution spectra.

Abstract

By comparison with high-resolution synchrotron x-ray experimental results, we assess several theoretical treatments for the bound-free (core-electron) contribution to x-ray Thomson scattering (i.e., also known as nonresonant inelastic x-ray scattering). We identify an often overlooked source of systematic error in the plane- wave form factor approximation (PWFFA) used in the inference of temperature, ionization state, and free electron density in some laser-driven compression studies of warm dense matter. This error is due to a direct violation of energy conservation in the PWFFA. We propose an improved practice for the bound-free term that will be particularly relevant for XRTS experiments performed with somewhat improved energy resolution at the National Ignition Facility or the Linac Coherent Light Source. Our results raise important questions about the accuracy of state variable determination in XRTS studies, given that the limited information content in low-resolution XRTS spectra does not strongly constrain the models of electronic structure being used to fit the spectra.