X-ray absorption spectroscopy for studying extreme conditions of pressure and temperatures

Studying matter under extreme conditions is critically important for a wide range of science application including, for example planetology and astrophysics research as well as industrial applications (laser peening, drilling...). Energy exchange between particles of different species, and more specifically electrons and ions as well as the subsequent equilibration of their temperatures plays a crucial role in many modern experiments creating High Energy Density (HED) matter. The interplay between electron and ions infers directly on the transport properties and is at the origin of exotic properties of matter. For example bond hardening, ultrafast transparency, complex and metastable crystallographic phases or even inversion of the melting curve have been observed. Recent studies have also underlined the major role of electrons on atomic structures at extreme pressures, demonstrating the existence of unexpected material properties at far extreme conditions. X-ray Absorption Near Edge Structure (XANES) is currently one of the best diagnostic for characterizing the electron – atomic interplay as it simultaneously probes the valence electron state and the local atomic arrangement. For that reason, XANES has now been demonstrated to be a powerful diagnostics for retrieving electron temperature, change of density of state and screening effect under strong ionization by observing the slope shape, shift of the edge and bandgap closure [Dorchies 2016, Denoeud 2016]. Because the typical time-scales of electronic structure transformation starts from femtosecond, ultrafast Time-Resolved X-ray Absorption Spectroscopy TR-XAS stands for a major diagnostics to further understand the dynamical properties of matter at extreme conditions. Here, I will briefly introduce XANES fundamentals and challenges and I will specifically detail its possibilities for HED science. I will also present a short overview of XANES experiments at laser facilities on Al and SiO2 samples as well as recent experiments that have been performed at the ESRF synchrotron and at the LCLS Free Electron Laser on Iron and Iron oxides.