X-ray diagnosis of the pressure induced Mott nonmetal-metal transition.

The evolution of the K-edge x-ray absorption near-edge spectroscopy (XANES) spectrum is investigated for an aluminum plasma expanding from the solid density down to 0.5  g/cm{3}, with temperatures lying from 5 down to 2 eV. The dense plasma is generated by nanosecond laser-induced shock compression. These conditions correspond to the density-temperature region where a metal-nonmetal transition occurs as the density decreases. This transition is directly observed in XANES spectra measurements through the progressive formation of a preedge structure for densities around 1.6  g/cm{3}. Ab initio calculations based on density functional theory and a jellium model have been efficiently tested through direct comparison with the experimental measurements and show that this preedge corresponds to the relocalization of the 3p atomic orbital as the system evolves from a dense plasma toward a partially ionized atomic fluid.