Optical second-harmonic generation from thallium on silver

Light which was generated at the surface of polycrystalline Ag was monitored in the specular direction at the second-harmonic energy of the neodynium-doped yttrium aluminum garnet laser incident radiation as a function of the applied voltage in a thin-layer electrochemical cell (0.05M ${\mathrm{H}}_{2}$${\mathrm{SO}}_{4}$) with and without the presence of 0.75\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$M ${\mathrm{Tl}}^{+}$. The second-harmonic generation (SHG) decreased when ${\mathrm{Tl}}^{+}$ became physisorbed prior to underpotential deposition (UPD) of a stable Tl monolayer and increased slightly upon UPD of Tl. In contrast to this, previously reported reflectance data at the first harmonic are insensitive to events preceding Tl deposition. These results point out basic differences between SHG and reflectance of the fundamental and also show that simple surface-charge-density arguments are unsatisfactory in the explanation of SHG surface phenomena.