Theoretical treatments of the bound-free contribution and experimental best practice in X-ray Thomson scattering from warm dense matter

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 planewave 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.

Observations of Continuum Depression in Warm Dense Matter with X-Ray Thomson Scattering

“…In this study, the atomic Lshell contribution was calculated in the Impulse Approximation (IA) [51] from the Hartree-Fock (HF) 2s wave function [52] as suggested to provide accurate results by a recent study in Ref. [53].…”

Thomson scattering from a three-component plasma

“…This need for accurate bound-free matrix elements for applications to plasma diagnostics was emphasized recently by Mattern and Seidler [15].…”

Characterizing plasma conditions in radiatively heated solid-density samples with x-ray Thomson scattering

“…The literature contains many examples in which the inelastic scattering feature is compared with theoretically generated fits to infer plasma properties such as electron density and temperature [4,23,24,26]. While the modeling behind the free-free scattering feature is thought to be well understood, the shape and scaling of the bound-free term can differ with different theoretical treatments [39,[43][44][45][46][47]. The bound-free term is of-ten modeled using the impulse approximation, which is known to be a reasonable approximation for the case of k → ∞ [28].…”

“…The bound-free term is of-ten modeled using the impulse approximation, which is known to be a reasonable approximation for the case of k → ∞ [28]. However, there remains uncertainty as to the proper scaling of the bound-free feature with respect to other features in the spectrum [43][44][45]. Because of such uncertainties, some recent work seeks to interpret scattering spectra outside of the Chihara decomposition, with methods like density functional theory [9,35,48].…”