Electron-ion thermal equilibration dynamics in femtosecond heated warm dense copper

Nonequilibrium warm dense copper produced by a femtosecond optical laser pulse is investigated with x-ray absorption near-edge spectroscopy. A laser-plasma-based tabletop device is set up to probe a pre-edge structure observed near the L edge with a picosecond resolution. Careful ab initio calculations are used to retrieve the electron temperature evolution up to equilibration with ions. Measurements are well reproduced by two-temperature model calculations based on temperature-dependent parameters, provided that the hydrodynamic expansion is taken into account. At the highest temperatures investigated (∼20000K), the last affects significantly the energy balance between electrons and ions.