Demonstration of an x-ray Raman spectroscopy setup to study warm dense carbon at the high energy density instrument of European XFEL

K Voigt,K Falk,L B Fletcher,M Harmand,M Zhang,T Döppner,D Kraus,A K Schuster,T Toncian,U Zastrau,S Göde,N J Hartley,R W Falcone,D O Gericke,D Chekrygina,E Brambrink,A Amouretti,Stefan P Hau‐Riege ,Oliver Humphries ,Mikako Makita ,A Pełka ,Mani Lokamani ,Valerio Cerantola ,Lingen Huang ,Kushal Ramakrishna ,Karen Appel ,Clemens Prescher ,Jan Vorberger ,Thomas R Preston ,Michal Šmíd

doi:10.1063/5.0048150

Abstract

We present a proof-of-principle study demonstrating x-ray Raman Spectroscopy (XRS) from carbon samples at ambient conditions in conjunction with other common diagnostics to study warm dense matter, performed at the high energy density scientific instrument of the European x-ray Free Electron Laser (European XFEL). We obtain sufficient spectral resolution to identify the local structure and chemical bonding of diamond and graphite samples, using highly annealed pyrolytic graphite spectrometers. Due to the high crystal reflectivity and XFEL brightness, we obtain signal strengths that will enable accurate XRS measurements in upcoming pump–probe experiments with a high repetition-rate, where the samples will be pumped with high-power lasers. Molecular dynamics simulations based on density functional theory together with XRS simulations demonstrate the potential of this technique and show predictions for high-energy-density conditions. Our setup allows simultaneous implementation of several different diagnostic methods to reduce ambiguities in the analysis of the experimental results, which, for warm dense matter, often relies on simplifying model assumptions. The promising capabilities demonstrated here provide unprecedented insights into chemical and structural dynamics in warm dense matter states of light elements, including conditions similar to the interiors of planets, low-mass stars, and other celestial bodies.

Highlights

We present a proof-of-principle study demonstrating x-ray Raman Spectroscopy (XRS) from carbon samples at ambient conditions in conjunction with other common diagnostics to study warm dense matter, performed at the high energy density scientific instrument of the European x-ray Free Electron Laser (European x-ray free electron lasers (XFELs))
Molecular dynamics simulations based on density functional theory together with XRS simulations demonstrate the potential of this technique and show predictions for high-energy-density conditions
We have presented x-ray Raman scattering spectra of ambient diamond and graphite recorded at the HED instrument of the European XFEL in an experimental setup well suited for future Warm dense matter (WDM) experiments

Summary

MOTIVATION

Warm dense matter (WDM), an extreme material state with temperatures on the order of several thousand to several million kelvin and solid densities, poses severe challenges for its characterization.[1]. F and the XRS spectrum becomes equivalent to the x-ray absorption spectrum.[35,36] At higher momentum transfer, non-dipole transitions can be probed.[34,42,43] In addition, the XRS intensity can be optimized to a certain extent by the choice of q, due to its quadratic dependence on the momentum transfer.[42] A first attempt of studying the carbon K-edge in non-collective scattering spectra of isochorically heated carbon (pyrolytic graphite) was performed in 2014 at the Linac Coherent Light Source (LCLS).[18] While edge-shifts due to ionization potential depression in dense matter could be observed in this experiment, the energy resolution of 20 eV, resulting from the spectrometer resolution and the XFEL bandwidth, did not allow the investigation of the substructures in the XRS signal. We show theoretical predictions using density functional theory molecular dynamics (DFT-MD) and XRS simulations demonstrating the potential for experiments using rep-rated drive systems

EXPERIMENT

SIMULATIONS

SUMMARY