XANES & EXAFS: principles, measurements and applications for matter in extreme conditions

    Different techniques using high-energy lasers and/or other short pulses are now routinely used now to dynamically bring matter up to extreme conditions. A large thermodynamic domain is accessible including the warm dense regime, with pressure easily exceeding the Mbar level. In order to get valuable data, time-resolved diagnostics are needed, since the process is eminently transient. Many results have been obtained in last decades, using optical diagnostics. The field has been recently upset by the development of X-ray diagnostics. As the most striking feature, X-rays can give information at the atomic-level scale, allowing direct comparison with first principles calculations such as ab initio QMD (Quantum Molecular Dynamics).
Among the different X-ray based techniques, X-ray Absorption Fine Spectroscopy (XAFS, including XANES – X-ray Absorption Near-Edge Spectroscopy – and EXAFS – Enhanced X-ray Absorption Fine Structure) is very well adapted to a large panel of extreme conditions. Indeed, the short-range order is probed without a crystalline order is required. In addition, valuable data are available on the “valence” electron structure that is responsible for all the macroscopic properties of matter (transport coefficients, equation of state…).
In this seminar, the principles of XAFS will be presented, as well as the experimental techniques recently developed to register exploitable X-ray absorption spectra through matter in extreme conditions. The potential of XANES and EXAFS will be illustrated by several recent studies involving measurements and state-to-the-art numerical simulations. They demonstrate, in particular, that: i) the electron temperature can be directly measured from the X-ray absorption edge slope in simple metals, ii) XANES provides direct observation of the electron structure modification, e.g. when Aluminum turns from metal to non-metal or inversely when SiO2 is metalized at high pressure, iii) EXAFS oscillations can track the short-range disordering from solid to warm dense matter.
In addition to applications in studies of matter under extreme conditions, time-resolved XAFS is a very powerful diagnostic to disentangle the dynamic interplay between electron structure and ion lattice, in non-equilibrium situations encountered when matter is heated by femtosecond pulse. Strong modifications of the matter properties are expected in such a regime where electrons are hot while the lattice is still cold (bond softening and/or bond hardening, depending on material).