A Measurement of the Electron-Ion Equilibration Rate in Warm Dense Gold with High-Resolution 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 of electron-ion interactions. We have implemented a high-resolution (~50 meV) X-ray scattering platform[3], designed for use with free-electron lasers, with a resolution capable of measuring the quasi-elastic Rayleigh peak. The width of this peak, which is essentially governed by Doppler broadening, provides 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 laser-excited gold films during the first 25 ps after irradiation, during which the ions are rapidly heated to electronvolt temperatures. The extracted electron-ion equilibration rate is compared to several theoretical and computational models.

[1] E. Bévillon et al. Physical Review B, 89(11):115117 (2014)

[2] T. G White et al. Physical Review B, 90(1):014305 (2014)

[3] E. E. McBride et al. Review of Scientific Instruments, 89(10):10F104, (2018)