Charge Transfer Excitations in SERS: Comparative Study of Benzene, Pyridine and Pyrazine

Abstract

We discuss a model of resonant Raman scattering by photon driven charge transfer between an electrode and an adsorbate as well as the influence of solvation and Coulomb relaxation on the energetic position of the electronic affinity levels. The potential dependence of the surface enhanced Raman (SER) intensity of the total symmetric skeleton modes at different laser photon energies is in qualitative agreement with the model. The observed energetic shift of the affinity levels is bigger than the applied shifts of potential, this is explained by local potential effects. The reversible static charge transfer to pyrazine is observed by a reversible change of the Raman spectrum. The experimental results cannot be understood with the “ground state CT model of SERS”. More advanced models of SERS by CT should comprise both aspects, “coherent tunneling” CT and “vibrational driven hopping” CT.