Model Independent Electron-Ion Equilibration Rate, Debye Temperature, and Bond Strength Measurements in Warm Dense Metals with Inelastic X-Ray Scattering

When a high-intensity laser is incident on 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 (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 platform
³
, designed for use with free-electron lasers, with a resolution capable of measuring changes to 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 of the plasma. We have measured the rise of the ion temperature in a variety of laser excited metallic thin films (Au, Ag, Cu, and Ti) over the first ~20 ps after irradiation, during which the ions are rapidly heated to electron volt temperatures. Useful quantities can be determined using the temporal evolution of the ion temperature; electron-ion equilibrations rates, Debye temperature, and Bond Hardening will be discussed and compared to several theoretical and computational models.