Experimental and numerical study of out-of-equilibrium properties in the warm dense matter regime of metals at the femtosecond timescale

The irradiation of materials using a femtosecond infrared laser brings matter in the so called warm dense matter regime, a frontier between solid state physics and plasma physics. At such a short timescale, an out-of-equilibrium situation occurs, where a great amount of energy is deposited on electrons, while the ions remains cold. This out-of-equilibrium regime may induce strong changes in the properties of the matter. This study focus on two different metals in the warm dense matter regime, copper and molybdenum. These metals have been studied experimentally using time-resolved X-ray absorption near edge spectroscopy (XANES), and numerically using ab initio simulations and hydrodynamics simulations. These simulations allow us to study matter respectively at the atomic scale through the electronic and ionic structures, and to the macroscopic scale by describing the behaviors of electrons and ions subsequent to an energy deposition. Amongst others, these three tools have been used to study copper extensively in the out-of-equilibrium warm dense matter regime. In particular, a diagnostic for the electronic temperature in copper have been developed, based on XANES spectroscopy near the L3 edge and ab initio simulations. Using these results, we studied the dynamics of the electron temperature in laser-heated warm dense copper at the femtosecond timescale, and highlighted that, beyond a certain laser flux, the electron energy transport transport is dominated by thermal diffusion rather than ballistic electron transport. Then, we transposed the methodology to study warm dense matter on the molybdenum, chosen to be the prototype for the study of transition metals in this regime. Hydrodynamics simulations allow us to estimate the typical thermodynamical conditions of molybdenum after a femtosecond laser pulse interaction. Ab initio simulations have then be conducted on warm dense molybdenum. Specifics features in the XANES spectra near the L3 edge of molybdenum have been identified, linked to the electronic and atomic structures, and their changes in the warm dense matter regime correlated to the temperature and density conditions. Finally, a preliminary XANES experiment conducted on warm dense molybdenum showed that the cold XANES spectra is in good agreement with the results of the ab initio simulations. Moreover, it also showed that it is possible to observe the changes in the spectra of molybdenum expected by the simulations in the warm dense regime. (author)