Intensity of Raman scattering from molecules adsorbed on a metallic grating

Abstract

A simple phenomenological model is presented for the calculation of intensity of Stokes scattered light due to vibrations of molecules adsorbed on a metallic grating or a rough metal surface. The surface roughness is assumed to be represented by a superposition of sinusoidal gratings of different periodicities, with each amplitude small compared to the wavelength of the incident light. Enhancements of the surface-plasmon field and the resulting induced surface charge density arising from resonant excitation of transverse surface plasmons are calculated explicitly. In our model, the net enhancement of the Raman cross section is shown to arise from two different types of contribution. The first contribution represents the usual mechanism involving the modulation of the molecular polarizability $\overleftrightarrow{\alpha }$ in the presence of the enhanced electromagnetic field at the molecular site, with the possibility of an additional enhancement due to the conversion of the near-zone Stokes field into the scattered radiation field by the metal surface. The second contribution comes from the modulation of the surface polarizability $\overleftrightarrow{\beta }$ arising from the finite spatial extent of the induced surface charge density at the surface. For vibrations of molecular monolayers perpendicular to the surface, it is shown that the additional polarizability contribution can become larger than the first contribution if the barrier potential for the conduction electron tunneling to the molecular site is less than about 1 eV.