Charge transfer effects in surface enhanced Raman scattering

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 1 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 (approximated as a simple cubic metal with tight binding wave functions constructed from Ag 5s valence orbitals), with charge-transfer excitation of an electron from the metal to the antibonding ethylene 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. The net enhancement is the product of the charge-transfer gain and the electrodynamic enhancement due to plasmon resonances at surface roughness elements. Symmetric vibrations usually will be enhanced substantially more than nonsymmetric ones by charge-transfer because, in contrast to non-resonant Raman scattering, the vibrational coupling is primarily Franck- Condon (due to differences in the equilibrium nuclear configurations of the ground and excited charge transfer states and the resulting nonorthogonality of different vibrational sublevels of these states) rather than Herzberg-Teller (due to vibrationally induced changes in the electronic wave functions). The charge-transfer mechanism is selective with the most enhanced vibrations involving those atoms which experience the greatest change in electron density between the ground and excited charge-transfer state. A recent report of SERS for benzene on platinum, 2 strongly suggests charge-transfer enhancement because the electromagnetic-field-enhancing plasmon resonances are strongly damped in this metal. The complete paper will be published in the December 1, 1982 issue of the Journal of Chemical Physics.