Surface enhanced Raman spectra: A critical review of the image dipole description

Abstract

Recent experiments have demonstrated enhancements of several orders of magnitude for Raman scattering of pyridine and other molecules adsorbed on a silver surface. One explanation which has been proposed is an image dipole model, in which a point dipole is mirrored in the metal, with the mirror image inducing an additional dipole in the adsorbed species and thus changing its effective polarizability α_eff. In this paper we suggest that the large enhancements obtained from such a point dipole description result from the use of an symptotic theory at unphysically short distances. We demonstrate this through a model calculation of α_eff of a hydrogen atom near a mirror surface. A point dipole approximation gives a divergent α_eff near a critical distance R_c from the surface, while a more exact self‐consistent Hartree calculation (including the finite extent of the hydrogen charge cloud) gives only a slight enhancement or even a de‐enhancement near R_c.