X-ray absorption spectroscopy of iron at multimegabar pressures in laser shock experiments

Abstract

Taking advantage of the new opportunities provided by x-ray free electron laser (FEL) sources when coupled to a long laser pulse as available at the Linear Coherent Light Source (LCLS), we have performed x-ray absorption near-edge spectroscopy (XANES) of laser shock compressed iron up to 420 GPa ($\ifmmode\pm\else\textpm\fi{}50$) and 10 800 K ($\ifmmode\pm\else\textpm\fi{}1390$). Visible diagnostics coupled with hydrodynamic simulations were used to infer the thermodynamical conditions along the Hugoniot and the release adiabat. A modification of the pre-edge feature at 7.12 keV in the XANES spectra is observed above pressures of 260 GPa along the Hugoniot. Comparing with ab initio calculations and with previous laser-heated diamond cell data, we propose that such changes in the XANES pre-edge could be a signature of molten iron. This interpretation then suggests that iron is molten at pressures and temperatures higher than 260 GPa ($\ifmmode\pm\else\textpm\fi{}29$) and 5680 K ($\ifmmode\pm\else\textpm\fi{}700$) along the principal Fe Hugoniot.