X-ray Thomson scattering spectra from DFT-MD simulations based on a modified Chihara formula

We study state-of-the-art approaches for calculating x-ray Thomson scattering spectra from density functional theory molecular dynamics (DFT-MD) simulations based on a modified Chihara formula that expresses the inelastic contribution in terms of the dielectric function. We compare the electronic dynamic structure factor computed from the Mermin dielectric function using an ab initio electron-ion collision frequency to computations using a linear response time dependent density functional theory (LR-TDDFT) framework for hydrogen and beryllium and investigate the dispersion of free-free and bound-free contributions to the scattering signal. A separate treatment of these contributions in the Mermin dielectric function shows excellent agreement with LR-TDDFT results for ambient-density beryllium, but breaks down for highly compressed matter where the bound states become pressure ionized. LR-TDDFT is used to reanalyze x-ray Thomson scattering experiments on beryllium demonstrating strong deviations from the plasma conditions inferred with traditional analytic models at small scattering angles.Comment: 14 pages, 10 figures, submitted to Physical Review

Carbon ionization at Gbar pressures: an ab initio perspective on astrophysical high-density plasmas

A realistic description of partially-ionized matter in extreme thermodynamic states is critical to model the interior and evolution of the multiplicity of high-density astrophysical objects. Current predictions of its essential property, the ionization degree, rely widely on analytical approximations that have been challenged recently by a series of experiments. Here, we propose a novel ab initio approach to calculate the ionization degree directly from the dynamic electrical conductivity using the Thomas-Reiche-Kuhn sum rule. This Density Functional Theory framework captures genuinely the condensed matter nature and quantum effects typical for strongly-correlated plasmas. We demonstrate this new capability for carbon and hydrocarbon, which most notably serve as ablator materials in inertial confinement fusion experiments aiming at recreating stellar conditions. We find a significantly higher carbon ionization degree than predicted by commonly used models, yet validating the qualitative behavior of the average atom model Purgatorio. Additionally, we find the carbon ionization state to remain unchanged in the environment of fully-ionized hydrogen. Our results will not only serve as benchmark for traditional models, but more importantly provide an experimentally accessible quantity in the form of the electrical conductivity.Comment: accepted for publication in Physical Review Researc

Evidence for phonon hardening in laser-excited gold using x-ray diffraction at a hard x-ray free electron laser.

Studies of laser-heated materials on femtosecond timescales have shown that the interatomic potential can be perturbed at sufficiently high laser intensities. For gold, it has been postulated to undergo a strong stiffening leading to an increase of the phonon energies, known as phonon hardening. Despite efforts to investigate this behavior, only measurements at low absorbed energy density have been performed, for which the interpretation of the experimental data remains ambiguous. By using in situ single-shot x-ray diffraction at a hard x-ray free-electron laser, the evolution of diffraction line intensities of laser-excited Au to a higher energy density provides evidence for phonon hardening

Evidence for phonon hardening in laser-excited gold using x-ray diffraction at a hard x-ray free electron laser

Studies of laser-heated materials on femtosecond timescales have shown that the interatomic potential can be perturbed at sufficiently high laser intensities. For gold, it has been postulated to undergo a strong stiffening leading to an increase of the phonon energies, known as phonon hardening. Despite efforts to investigate this behavior, only measurements at low absorbed energy density have been performed, for which the interpretation of the experimental data remains ambiguous. By using in situ single-shot x-ray diffraction at a hard x-ray free-electron laser, the evolution of diffraction line intensities of laser-excited Au to a higher energy density provides evidence for phonon hardening

Dataset: Measuring the structure and equation of state of polyethylene terephthalate at megabar pressures

This repository contains raw-data related to our publication "Measuring the structure and equation of state of polyethylene terephthalate at megabar pressures". The XRD data in the "LCLS" folder is accompanied with a "calibration.poni" file that provides information about the experiment's geometry and can be used in pyFAI (GitHub page) or Dioptas (GitHub page) to integrate the two-dimensional data azimuthally. Integrated XRD data after background-subtraction and filter-corrections is presented in Fig. 2 and 3 of the manuscript while 2D data of run 215 is used in Fig. 1. The "shotlist.csv" file contains information about the relative X-ray to drive-laser timing, shot-type and X-ray energy for the individual events. VISAR, SOP and reflectivity measurements can be found in the "LULI" directory. 2ω-VISAR and SOP datasets of shot 08 are displayed as inserts in Fig. 5 (the first after performing a ghost-fringe subtraction). "shotlist.csv" provides additional parameters. The DFTMD folder contains the results of our density functional theory molecular dynamics simulation. In the "XRD" subdirectory, "wrofk_mylar_chomd*.dat" files can be found in which the quantities to calculate the lineouts in Fig. 3 and 4 are saved for given temperatures, pressures and densities. The header of those files is given in "header.txt" and additional information about the conditions and settings for individual calculations can be obtained from "param_mylar_md.txt". The dataset for the Hugoniot curve from our DFT-MD equation-of-state (which is plotted in Fig. 5) is provided in the "Hugoniot" sub-folder