Limit on short-range resonant-charge-transfer contribution to surface-enhanced Raman scattering

The magnitude of the contribution of chemical effects to the enhancement of Raman scattering from adsorbates on a metal surface and its experimental identification are considered. Within a simple model for the electronic structure of the surface, local enhancements in the range ${10}^2$-${10}^3$ can be obtained. The unambiguous identification of these contributions requires a detailed characterization of the electromagnetic contributions to the enhancement of the Raman intensity.