Visible-infrared difference frequency generation at CN- covered Au

We develop a model for calculating the optical difference frequency generat ion (DFG) spectra of adsorbate covered surfaces, and apply it to analyze re cently obtained results for CN- covered Au. We split the surface nonlinear susceptibility of the system into separate contributions from the noncentro symmetric adsorbate layer and from the substrate. The former has a dipolar susceptibility given by a product of the Raman and infrared responses of it s stretch mode. To account for the virtual interband transitions of the sub strate we employ a continuous dipolium model immersed within a semiinfinite jellium which allows for the intraband screening. The resulting DFG line s hape depends on the relative phase between the adsorbate and the substrate responses and yields information on the orientation of the molecule and on the strength of its effective dipole moment.