One of the most widely debated questions concerning surface-enhanced Raman scattering (SERS) is the nature of the surface morphology required to obtain the large enhancements. While it is generally agreed that surface roughness is important, the specific nature of this roughness, and its exact role in the enhancement has been a subject of considerable debate. In this chapter, we discuss experimental results that address this question directly. We study an extremely low frequency Raman mode which we attribute to Raman scattering (RS) from the rough metal surface itself.^1,2 From the low frequency Raman scattering we can determine the nature of the roughness which contributes to SERS. From the anomalous behavior of this Raman scattering, we can determine the role of this roughness in the large enhancement of the RS cross section of adsorbed molecules.

Both the spectral dependence and the magnitude of the excitation spectra of surface-enhanced Raman scattering are found to be quantitatively related to the absorption spectra for silver-island films. This relationship is maintained even when the local electromagnetic enhancement at the islands is varied by coating the film with a layer of absorbing dye molecules. The results illustrate a simple experimental way to determine independently the role of the electronic plasma resonances in the enhancement process on these films and shed new insights into the details of the electromagnetic interactions responsible for surface-enhanced Raman scattering.

The fluorescence of molecules on silver-island films exhibits nonexponential decay and is several orders of magnitude more rapid than on a silica surface, while the total emission intensity is slightly increased. This behavior results from the electromagnetic coupling between the fluorescing molecules and the plasmon resonances of the silver islands. We obtain good agreement with a model that uses the same theory for the electromagnetic coupling as has been used to account for surface-enhanced Raman scattering.

We have examined how the photochemical reaction rate of molecules absorbed at rough silver surfaces is affected by the same strong electrodynamic interactions which lead to surface-enhanced Raman scattering. We probe the effects of the spatial variation of the electrodynamic interactions across the island film.