Electronic mechanism of the surface enhanced Raman scattering

Electronic mechanism of the surface enhanced Raman scattering (SERS) is investigated by an ab initio molecular orbital theory. The time-dependent Hartree-Fock method is used to calculate the polarizability of the surface-molecule interacting system. For representing the surface effect, we use small solid clusters Ag₂, Ag₁₀, K₂, Pd ₂, and MgO. The present method succeeds in describing the enhancement of the Raman intensity for the adsorbed CO molecule. The maximum intensity of the Ag₁₀CO system is calculated to be seven orders of magnitude larger than that of the free CO molecule. Furthermore, the wavelength dependence of the Raman intensity calculated by the Ag₁₀CO system agrees with the experimental spectrum. The electronic mechanism of the SERS is due to the resonance transitions, in which the surface polarization and the surface-molecule interaction are very important. This mechanism explains the orientation- and distance-dependencies in the surface-adsorbate interacting systems.