Ab initio modeling of nonequilibrium electron-ion dynamics of iron in the warm dense matter regime

Abstract

The spatiotemporal electron and ion relaxation dynamics of iron induced by femtosecond laser pulses was studied using a one-dimensional two-temperature model (1D-TTM) where electron and ion temperature-dependent thermophysical parameters such as specific heat ($C$), electron-phonon coupling ($G$), and thermal conductivity ($K$) were calculated with ab initio density-functional-theory (DFT) simulations. Based on the simulated time evolutions of electron and ion temperature distributions [${T}_{e}(x,t)$ and ${T}_{i}(x,t)$], the time evolution of x-ray absorption near-edge spectroscopy (XANES) was calculated and compared with experimental results reported by Fernandez-Pa\~nella et al., where the slope of XANES spectrum at the onset of absorption ($s$) was used due to its excellent sensitivity to the electron temperature. Our results indicate that the ion temperature dependence on $G$ and $C$, which is largely neglected in the past studies, is very important for studying the nonequilibrium electron-ion relaxation dynamics of iron in warm dense matter (WDM) conditions. It is also shown that the $1/s$ behavior becomes very sensitive to the thermal gradient profile, in other words, to the values of $K$ in a TTM simulation, for target thickness of about two to four times the mean free path of conduction electrons. Our approach based on 1D-TTM and XANES simulations can be used to determine the optimal combination of target geometry and laser fluence for a given target material, which will enable us to tightly constrain the thermophysical parameters under electron-ion nonequilibrium WDM conditions.