Probing extreme states of matter using high-resolution inelastic X-ray scatterging at X-ray free electron lasers.

The use of ultra-short, ultra-bright X-ray pulses generated at X-ray Free Electron Laser (XFEL) light sources, when coupled with high-intensity optical laser, has enabled the investigation of high-temperature, high-pressure states of matter, often referred to as warm dense matter. In particular, inelastic X-ray scattering has proven to be a powerful technique to probe this exotic state of matter. Here I will introduce high-resolution inelastic X-ray scattering and describe its use in investigating warm dense matter.

I will focus warm dense matter generated via laser-compression techniques, for which, direct and accurate measurements of thermodynamic and transport properties are essential for understanding the behavior of matter at extreme pressures and temperatures. While X-ray diffraction measurements have allowed in situ measurement of structure and density, the direct measurement of temperature remains a challenge. In shock compression experiments, it is often estimated from hydrodynamic simulations or inferred using streaked optical pyrometry, which requires a priori knowledge of the material properties at extreme conditions. Here, I will describe experiments conducted at the Linac Coherent Light Source using high-resolution inelastic X-ray scattering to access density, temperature, and particle velocities. With the installation of high repetition rate drive lasers, this technique has the potential to become a powerful tool to investigate properties of shock-compressed matter.