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 x 10(12)-5 x 10(13) W/cm(2). We measured time-resolved p- and s-polarized reflectivity (r(p) and r(s)) and the s-p phase difference (delta). When plotted as Y=2|r(s)||r(p)|sin((delta)]/(|r(s)|(2)+|r(p)|(2)) versus X=|r(p)|(2)/|r(s)|(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)x 10(-24) cm(3). 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.