REFLECTIVITY AND OPTICAL BRIGHTNESS OF LASER-INDUCED SHOCKS IN SILICON

We report the first simultaneous measurement of the reflectivity and o ptical emission of a strong (48 Mbar) shock front emerging at a free s urface of a solid. Planar shock waves were driven by thermal x rays fr om a laser-heated cavity. The inferred model-independent brightness te mperature of the shock front in silicon turns out to be significantly below the expected Hugoniot temperature. We find that our data cannot be explained within the two-temperature model which assumes instantane ous metallization of silicon in the density jump.