Raman scattering of dye molecules adsorbed on metal surfaces. Theoretical considerations

We discuss the Raman scattering of dye molecules adsorbed on metal (surface enhanced Raman scattering (SERS) active) surfaces. We show that surface resonance enhancement models (RE) predict that the surface enhancement of dyes will be orders of magnitude smaller than for molecules, which in solution are not in resonance with the incident light. On the other hand, local field and emission enhancement mechanisms (LFE) require similar enhancement factors for dyes with broad excitation profiles and for nonchromophoric molecules. Most experiments exhibit very different surface enhancements for these two "types" of molecules, indicating that RE mechanisms provide the major contribution to the total SERS. LFE mechanisms are, therefore, limited to a more minor role. We also discuss the enhancement hierarchy of the resonance Raman process and relaxed fluorescence and show that the (scarce) experimental data are inconsistent with the predictions of the LFE mechanism.