Ultrashort-pulse laser pump-probe ellipsometry has been performed on gold targets at intensities $2\times {10}^{12}–5\times {10}^{13}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$. We measured time-resolved $p$- and $s$-polarized reflectivity ($r_p$ and $r_s$) and the $s\mathrm{\text{−}}p$ 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 $X\mathrm{\text{−}}Y$ space, even for different pump intensities. Although the input energy density is about $40\mathrm{e}\mathrm{V}/\mathrm{\text{atom}}$ and the plasma expansion is rapid (up to $10\mathrm{k}\mathrm{m}/\mathrm{s}\mathrm{e}\mathrm{c}$), our data are consistent with a partially transparent blowoff having an atomic polarizability $(-1.75+0.2i)\times {10}^{-24}{\mathrm{c}\mathrm{m}}^3$. 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.

• Received 25 February 2003

DOI:https://doi.org/10.1103/PhysRevLett.91.075004