Normal (Unenhanced) Raman Scattering from Pyridine Adsorbed on the Low-Index Faces of Silver
Despite several years of intense investigation by a number of laboratories, the origin of surface-enhanced Raman scattering (SERS) remains the subject of a lively debate. Several review articles and a book summarize the current state of thinking on the subject [1–3]. The debate focuses upon the relative importance of two classes of enhancement mechanisms, which are termed electromagnetic (classical) and molecular (sometimes referred to as chemical or local). In the former, enhanced scattering arises from an increase in the local electric field of the exciting light at metal surfaces of high curvature, e.g., submicroscopically rough surfaces or diffraction gradings. In the latter, the enhancement is thought to arise from an increase in the molecular polarizability derivative that could occur when a molecule is chemisorbed to a metal surface. New optical transitions could result from charge transfer excitations which may be resonant with the exciting laser frequency, thus leading to resonance Raman scattering. Although there is general agreement as to the existence of both classes of enhancement mechanisms, the separation of their relative contributions, even for the best-studied case, pyridine on silver, remains elusive.
KeywordsAnisotropy Pyridine Auger Reso Chemisorption
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