Physisorption on a metal surface probed by surface state resonant second harmonic generation

Second harmonic generation from a Ag(1 1 0) surface, resonantly enhanced by the surface state transition at 1.74 eV, is found to be greatly affected by submonolayer adsorption. The physisorption of water or methanol causes a monotonic, exponential-like decay of the SH intensity that can be described by a model treating the adsorbate as a delocalized, weak perturbation in the resonantly enhanced SHG. On the other hand weak chemisorption of aniline generates a complex response in the SH intensity that eludes the predictability of the model. Analysis of the SH intensity has determined that water or methanol adsorption causes an upward shift in the minimum energies of the pair of surface states on Ag(1 1 0) and an increase in the transition linewidth. The sensitive response of the surface states to the presence of adsorbates provides a basis for SHG resonantly enhanced by surface state transition as a highly sensitive probe of submonolayer coverage.