XANES – S. Mazevet

fig-fex — Iron phase diagram: Our results (red open squares for solid hcp and red full square for liquid Fe) are compared with previous results and extrapolation obtained in the literature from diamond-anvil-cell compression [4, 6, 36], shock-waves experiments [8, 9, 10, 11, 12, 13], and ab initio simulations [1, 2, 3]. The ICB pressure of 330 GPa is shown with a dashed blue line.

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 (±50) and 10 800 K (±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 (±29) and 5680 K (±700) along the principal Fe Hugoniot.

http://journals.aps.org/prb/abstract/10.1103/PhysRevB.92.014105

fig1 — Electronic temperature Te extracted from the K-edge slope as a function of the temperature T. Symbols are similar to Fig. 2: (triangles) measurements, (circles) QMD calculations, and (squares) FD calculations. T is set to the value deduced from optical diagnostics (experiment) and to the calculation input values, respectively. Te and T are normalized to the Fermi energy EF estimated from the FEG model.

The use of the x-ray absorption K-edge slope is investigated as a model-free diagnostic of the electronic temperature in warm dense matter. Data are reported for aluminum in a wide domain of densities (approximately one to three times the solid density) and temperatures (∼0.1–10eV). Measurements are obtained from laser-sock co
mpression where both temperature and density are independently determined from optical diagnostics. They are compared with two different theoretical approaches, respectively, based on quantum molecular dynamics and multiple scattering. Extrapolation for other absorption edges and materials is discussed.