Mapping the Electronic Structure of Warm Dense Nickel via Resonant Inelastic X-ray Scattering*

The exploration of the quantum behaviour of high energy-density systems is a research area of broad importance to applications in plasma physics, astrophysics and fusion energy science. However, measuring the electronic structure in such systems remains a formidable experimental challenge, leading to a dearth of data of sufficient quality for benchmarking theoretical models in extreme plasma conditions. Resonant inelastic x-ray scattering (RIXS) is a popular experimental technique to study low-energy excitations in quantum systems, however, x-ray source requirements have so far limited its use to condensed matter systems. Here we show how the electronic structure of solid-density nickel heated to temperatures of 10’s of eV can be mapped via RIXS using the Linac Coherent Light Source free-electron laser at SLAC. We extract single-shot simultaneous measurements of electronic temperature, ionization, ionization potential depression, bound state energies and of the valence density of states. This technique provides a promising approach to observe directly electron relocalization as a function of the plasma environment for the first time.