Time-resolved study of hot dense germanium byL-shell absorption spectroscopy

We have used time-resolved 2p-3d absorption spectroscopy to probe the ionization degree of a germanium plasma heated by the soft x rays emitted from the rear side of laser-irradiated gold targets. We have determined the average ionization degree and the distribution of ion stages as a function of time during the expanding, cooling phase of the plasma. They have been compared with collisional-radiative distributions based on hydrodynamic simulations of the plasma parameters. Results show a slow decrease of the ionization degree from 〈Z〉=11.5 to 10.0 during a 3-ns period after gold heating. Comparison with theoretical spectra show matter densities around 0.01 g/cm³ and temperatures of about 50 eV.