Ultrashort-Pulse Laser Ellipsometric Pump-Probe Experiments on Gold Targets

Ultrashort-pulse laser pump-probe ellipsometry has been performed on gold targets at intensities 2×10¹²--5×10¹³ W/cm². We measured time-resolved p- and s-polarized reflectivity (rₚ and rₛ) and the s-p phase difference (δ). When plotted as Y=[2|rₛ||rₚ|sin(δ)]/(|rₛ|²+|rₚ|²) versus X=|rₚ|²/|rₛ|², the experimental data follow approximately the same curve in X-Y space, even for different pump intensities. Although the input energy density is about 40 eV/atom and the plasma expansion is rapid (up to 10 km/sec), our data are consistent with a partially transparent blowoff having an atomic polarizability (-1.75+0.2i)×10^-24 cm³. We attribute this behavior to recombination in the expanding plasma. The Saha equation predicts recombination and formation of negative ions and a low density of free electrons in the low-density vapor, and solutions of the Maxwell equations approximately reproduce the data.