Tracing X-ray-induced formation of warm dense gold with Boltzmann kinetic equations

Beata Ziaja,John Jasper Bekx,Nikita Medvedev,Michal Stransky,Vikrant Saxena,Sven Toleikis,Martin Masek,Przemyslaw Piekarz

doi:10.1140/epjd/s10053-021-00235-z

Abstract

In this paper, we report on the Boltzmann kinetic equation approach adapted for simulations of warm dense matter created by irradiation of bulk gold with intense ultrashort X-ray pulses. X-rays can excite inner-shell electrons, which triggers creation of deep-lying core holes. Their relaxation, especially in heavier elements such as gold (atomic number Z= 79) takes complicated pathways, involving collisional processes, and leading through a large number of active configurations. This number can be so high that solving a set of evolution equations for each configuration becomes computationally inefficient, and another modeling approach should be used instead. Here, we use the earlier introduced ’predominant excitation and relaxation path’ approach. It still uses true atomic configurations but limits their number by restricting material relaxation to a selected set of predominant pathways for material excitation and relaxation. With that, we obtain time-resolved predictions for excitation and relaxation in X-ray irradiated bulk of gold, including the respective change of gold optical properties. We compare the predictions with the available data from high-energy-density experiments. Their good agreement indicates ability of the Boltzmann kinetic equation approach to describe warm dense matter created from high-Z materials after their irradiation with X rays, which can be validated in future experiments.Graphic

Highlights

Excited matter created after intense X-ray radiation is an object of intense experimental studies with high power laser sources, in particular, with free electron lasers (FELs), see e.g., [7,9,21]
We report on the Boltzmann kinetic equation approach adapted for simulations of warm dense matter created by irradiation of bulk gold with intense ultrashort X-ray pulses
We compare the predictions with the available data from high-energy-density experiments. Their good agreement indicates ability of the Boltzmann kinetic equation approach to describe warm dense matter created from high-Z materials after their irradiation with X rays, which can be validated in future experiments

Summary

Introduction

Excited matter created after intense X-ray radiation is an object of intense experimental studies with high power laser sources, in particular, with free electron lasers (FELs), see e.g., [7,9,21]. For the simulation of the early non-equilibrium stages of the sample evolution, full kinetic equations should be applied They follow sample evolution during this stage, delivering the information on transient electron and ion distribution. Such equations should treat every active atomic configuration appearing during sample excitation and relaxation. In [32], we introduced an alternative approach which still uses true atomic configurations but limits their number by restricting the sample relaxation to the predominant excitation and relaxation pathways Applying this approach in what follows, we will obtain time-resolved predictions for excitation and relaxation of X-ray-irradiated bulk of gold, including the change of gold optical properties in response to an X-ray pulse.

Description of Boltzmann equation solver

Results for electronic and optical properties of X-ray irradiated gold

Conclusions and outlook