Electromagnetic contributions to single-molecule sensitivity in surface-enhanced Raman scattering

We examine whether single molecule sensitivity in surface-enhanced Raman sc attering (SERS) can be explained in the framework of classical electromagne tic theory. The influence of colloid particle shape and size, composition ( Ag or Au) and interparticle separation distance on the wavelength-dependent SERS enhancement factor is reported. Our calculations indicate that the ma ximum enhancement factor achievable through electromagnetics is of the orde r 10(11). This is obtained only under special circumstances, namely at inte rstitial sites between particles and at locations outside sharp surface pro trusions. The comparative rarity of such sites, together with the extreme s patial localization of the enhancement they provide, can qualitatively expl ain why only very few surface sites seem to contribute to the measured sign al in single-molecule SERS experiments. Enhancement factors of the order 10 (14)-10(15), which have been reported in recent experiments, are likely to involve additional enhancement mechanisms such as chemisorption induced res onance Raman effects.