The very large Raman scattering cross-section of adsorbed molecules, such as pyridine and CN-, on Ag is attributed to mechanisms involving either electron-hole pair excitations in the surface region of the metal or charge-transfer excitations between the metal substrate and the adsorbed molecules.

The giant RS by pyridine and CN− on Ag is accompanied by a strong RS continuum which is attributed to inelastic light scattering by charge carrier-excitations. The enhanced RS by the adsorbed molecules and by the charge carrier-excitations are attributed to surface roughness enhanced EM fields at the metal surface resulting from the excitation of transverse collective electron-excitations and surface-EM modes, and to surface roughness-induced radiative-excitation and radiative-recombination of particle-hole pairs.

The observation by Fleischmann, et al in 19741 of a strong Raman scattering (RS) in the visible by pyridine molecules adsorbed on a Ag electrode that had been roughened electrochemically by multiple oxidation-reduction cycles to increase the surface area, and the subsequent demonstration in 1977 by Jeanmaire and Van Duyne2 and by Albrecht and Creighton3 that the RS by pyridine molecules adsorbed on a Ag electrode following a single electrochemical oxidation-reduction cycle is greater by a factor of 105 to 106 than that of pyridine molecules in neat pyridine or in aqueous solution, has attracted widespread attention of theorists and experimentalists. Strongly enhanced RS has since then been observed for CN− and a number of other molecules adsorbed on Ag and an appreciably weaker RS has also been observed for molecules adsorbed on Cu, Pt and, more recently, on Au4–19.