Reflectivity and Optical Brightness of Laser-Induced Shocks in Silicon

We report the first simultaneous measurement of the reflectivity and optical emission of a strong (4--8 Mbar) shock front emerging at a free surface of a solid. Planar shock waves were driven by thermal x rays from a laser-heated cavity. The inferred model-independent brightness temperature 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 instantaneous metallization of silicon in the density jump.