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

When a high-intensity laser beam strikes a solid target, it induces a highly non-equilibrium state by rapidly heating one subsystem more than the other, leading to preferential heating dynamics^1;2These transient and high-energy-density plasmas serve as precursors to warm dense matter (WDM) and serve as an experimental platform for validating quantum mechanical theories regarding electron-ion interactions. To facilitate our research, we have developed a high-resolution X-ray scattering platform³ with an impressive resolution of approximately 50 meV. This platform is specifically designed for use with free-electron lasers, enabling us to precisely measure changes in the quasi-elastic Rayleigh peak. The width of this peak directly reflects the velocity distribution of ions and is primarily influenced by Doppler broadening. This measurement provides us with a model-independent assessment of the plasma's ion temperature. In the case of a thin metallic gold film, we've tracked the temporal evolution of ion temperature within the initial ~20 picoseconds following laser irradiation. During this brief timespan, the ions experience rapid heating, reaching electronvolt-level temperatures. Analyzing this evolution of ion temperature allows us to determine the equilibration process between electrons and ions within this unique regime. In our discussion, we will delve into the phase-dependent nature of this equilibration rate and explore the distinctive behavior that emerges near the solid-liquid phase boundary.

[1] E. Bevillon et al., Phys. Rev. B. 89(11), 115117 (2014)

[2] T. G. White et al., Phys. Rev. B. 90(1), 014305 (2014)

[3] E. E. McBride et al., Rev. Sci. Instrum. 89(10), 10F104 (2018)