Model for optical second-harmonic generation from centrosymmetric molecules adsorbed on a surface

Recently, second-harmonic generation has been observed from centrosymmetric molecules adsorbed on a silver surface. The second-harmonic generation is greatly enhanced under conditions which are similar to those under which the surface-enhanced Raman scattering is observed. In this Communication a model is presented which may account for the observed second-harmonic generation and gives an estimate of the effective second-order nonlinear polarizability in good agreement with experiment.