Visible-infrared sum and difference frequency generation at adsorbate-covered Au

We develop a model for sum and difference frequency generation (SFG/DFG) at the interface of an isotropic centrosymmetric nonsimple metal covered by a noncentrosymmetric adsorbate. We separate the nonlinear response of the su rface into a free-electron contribution that characterizes the intraband tr ansitions, a bound-electron contribution that characterizes the interband t ransitions, and a cross term due to the interaction among free and bound el ectrons. For the former we introduce an adjustable parameter consistent wit h semi-infinite jellium calculations, whereas for the others we apply a scr eened dipollium model that gives the nonlinear response in terms of the bou nd- and free-electron contributions to the substrate's bulk dielectric func tion. The nonlinear response of the adsorbate is described through the Rama n and IR response of its stretch mode. We calculate SFG/DFG spectra for a C N- covered Au-electrolyte interface choosing the direction of the dipole mo ment of the adsorbate either toward or away from the surface. The interfere nce between the substrate's and overlayer's responses allows the extraction of orientational information from a comparison of our model calculations t o recently obtained experimental spectra. [S0163-1829(99)12935-7].