Measuring the Electron-Ion Equilibration Rate in Warm Dense Metals with High-Resolution Inelastic X-ray Scattering

When a high-intensity laser hits a solid target, the preferential and rapid heating of one subsystem over the other creates a highly non-equilibrium state [1,2]. These transient, high-energy-density plasmas are a precursor to warm dense matter and serve as a testbed where we can validate quantum mechanical theories of electron-ion interactions. We have implemented a high-resolution (∼50meV) x-ray scattering platform [3], designed for use with free-electron lasers, with a resolution capable of measuring the quasi-elastic Rayleigh peak. Essentially governed by Doppler broadening, the peak's width is a direct measurement of the ions' velocity distribution and corresponds to a model-independent ion temperature measurement. We have measured the rise of the ion temperature in a variety of laser-excited metals (Au, Ag, Cu, and Ti) over the first ∼20 ps after irradiation, during which the ions are rapidly heated to electronvolt temperatures. The extracted electron-ion equilibration rates are compared to several theoretical and computational models.