Charge transfer effects in surface-enhanced Raman scatteringa)

Abstract

Surface-enhanced Raman scattering (SERS) due to charge-transfer interactions between the adsorbed molecule and the metal surface is analyzed using the semiempirical Wolfsberg–Helmholz method to relate the molecule–surface interactions and the resulting charge-transfer states to the overlap integrals between the metal conduction-band orbitals and an acceptor or donor molecular orbital of the molecule. Calculations for the model system of ethylene adsorbed on silver, with charge-transfer excitation of an electron from the metal to the antibonding etylene π orbital, show that charge-transfer Raman enhancements of the order of 10 to 1000 are possible if the charge-transfer band is partially resonant with the exciting radiation. Symmetric vibrations usually will be enhanced substantially more than nonsymmetric ones by this mechanism because the vibrational coupling is primarily Franck–Condon rather than Herzberg–Teller. The presence of overtone and combination bands in charge-transfer-enhanced Raman spectra is also possible.