VOLUME 62, NUMBER 14 PHYSICAL REVIEW LETTERS 3 APRIL 1989 Shock-Induced Shifts in the Aluminum K Photoabsorption Edge L. DaSilva, A. Ng, B. K. Godwal, (a) G. Chiu, and F. Cottet (b) Physics Department, University of British Columbia, Vancouver, British Columbia, Canada V6T2A6 M. C. Richardson and P. A. Jaanimagi Laboratory for Laser Energetics, University of Rochester, Rochester, New York 14627 Y. T. Lee Lawrence Livermore National Laboratory, University of California, Livermore, California 94550 (Received 25 July 1988) We present results of studies on shock-induced shifts in the K photoabsorption edge of aluminum. Time-resolved x-ray spectroscopic measurements indicated maximum red shifts of — 7 eV for compres- sions of —2.2 times the normal density. The results are interpreted using hydrodynamic simulations which incorporated a new model for calculating the A^-edge energy in a dense plasma. PACS numbers: 52.35.Tc, 52.50.Jm, 62.50.4-p, 78.70.Dm In recent years, high-power lasers have been found to have a growing use in research at high pressure and den- sity. In particular, Hugoniot studies have focused on the measurement of shock speed 1 " 5 and temperature. 5 These, however, yielded little information on the ioniza- tion state or electronic structure of the dense matter. Most recently, Bradley et al. 6 reported on the first exper- iment to probe the electronic structure of radiatively- heated and shock-compressed KC1 by measuring the photoabsorption edge shift of chlorine in a multilayered target. In this paper, we present measurements of shock-induced shifts in the aluminum K edge in laser- irradiated aluminum foils. Aluminum was chosen be- cause its high-pressure equation of state is well estab- lished. 7 The use of a uniform target allows us to follow the change in the K edge as the shock pressure is varied during the laser pulse. Most importantly, shock propa- gation will not be complicated by impedance mismatches between different target layers and the shock can be characterized by transit time measurements. Further- more, using thick targets and long pulses, we can access shock-compressed aluminum without radiative preheat. A new model for calculating the AT-edge energy was used in coupled radiation-hydrodynamic simulations to deter- mine the edge shift due to shock compression. The pre- dictions showed good qualitative agreement with the measurements. In the experiment, aluminum foils were irradiated with a 0.53-jum, 2.3-ns (FWHM) laser pulse. For the absorption spectroscopy measurement, 25-^m aluminum targets were chosen so that the shock would reach the target rear surfaces at a time near peak laser intensity. The laser beam was focused onto the target with // 5 op- tics at normal incidence. The intensity distribution at focus approximated a trapezoidal profile with 80% of the laser energy contained in a spot of ^-80 fim diam. As the shock wave was observed to emerge from the target- free surface in a region of —80 jum diam, we considered the effective irradiance to be the averaged absorbed irra- diance in this focal spot region. This was ~2.3xl0 13 W/cm 2 . To determine the resulting shock trajectory, shock transit times through 25-53-jum aluminum foils were measured from observations of luminous emission at target-rear surface. Details of this diagnostic have been described previously. 5 The results are presented in Fig. 1. These show good agreement with one-dimen- sional hydrodynamic simulations 8 which included multi- group radiation transport. 9 Thus, a good estimate of the pressure and density in the target as a function of time could be made using a known equation of state. 10 The calculated peak shock pressure was 3.5 Mbar which yielded a compression of 2.2. To determine the K edge in the shocked material, x- f 1 1 1 1 1 ' 0 10 20 30 40 50 POSITION (jjm) FIG. 1. Laser-driven shock wave in aluminum. Results from transit time measurements (circles) and one-dimensional simulations (solid curve). Also indicated is the calculated tra- jectory of the target layer giving rise to the most red shifts in the aluminum K edge (dashed curve). The zero corresponds to the time of peak laser intensity. 1623