Surface roughness and the enhanced intensity of Raman scattering by molecules adsorbed on metals

Greatly enhanced Raman scattering from monolayers of pyridine adsorbed on silver has been reported by several authors. I propose that the anomalous intensity arises from preresonant or resonant excitations of conduction electron resonances in adsorbate covered metal bumps on the surface. The bumps form a two-dimensional colliodlike layer which displays a collective resonance at a frequency which depends on the bump density. The model can account for the disparate excitation functions reported by two groups in terms of varying degrees of roughness. The apparent lack of enhancement with metals other than Group Ib is also explained.