Electron-Ion Equilibration Rates Across the Solid-Liquid Phase Boundary in Warm Dense Gold*

When a high-intensity laser is incident on a solid target, a highly non-equilibrium state is created through the process of preferential and rapid heating of one subsystem over the other^1;2. These transient, high-energy-density plasmas act as a precursor to warm dense matter (WDM) and serve as a testbed where we can validate quantum mechanical theories for electron-ion interactions. We have implemented a high-resolution (∼50meV) X-ray scattering platform³, designed for use with free-electron lasers, with a resolution capable of measuring changes to the quasi-elastic Rayleigh peak. The peak’s width is a direct measurement of the ions’ velocity distribution, essentially governed by Doppler broadening, which corresponds to a model-independent ion temperature measurement of the plasma. For a metallic thin gold film, we have measured the temporal evolution of the ion temperature over the first ∼20 ps after irradiation; in this time the ions are rapidly heated to electronvolt temperatures. The ion’s temperature evolution is used to determine the electron-ion equilibration in this regime. We will discuss the phase dependent nature of the equilibration rate and the unique behavior around the solid-liquid phase boundary.