Complete analysis of the surface-enhanced Raman scattering of pyrazine on the silver electrode on the basis of a resonant charge transfer mechanism involving three states

A new general procedure to interpret surface-enhanced Raman scattering (SERS) spectra has been developed in order to clarify the controversy concerning the relevant enhancement mechanism of this type of spectra. The analysis consists of detecting the presence of the charge transfer (CT) enhancement mechanism by correlating the most enhanced SERS bands with the ab initio calculated geometries (Delta Q) and vibrational frequencies (Delta v) of the isolated molecule and its radical anion. This CT mechanism is assumed to be identical to that of resonance Raman between the electronic ground state of the metal-adsorbate complex and charge transfer excited states. We consider that these excited states arise when one electron is transferred from the metal to pyrazine. For this reason, they have been labeled from the point of view of pyrazine on the basis on the symmetry of the doublet states of its radical anion. The SERS spectra of pyrazine recorded on silver surface at several electrode potentials have been analyzed on the basis of the Franck-Condon and Herzberg-Teller contributions related to B-2(3u)-(1)A(g) and (2)A(u)-(1)A(g) transitions. A great deal of experimental facts related to the relative enhancement of in-plane A(g), B-3g, and B-1u modes as well as out-of-plane B-3u, B-2g, and A(u) vibrations has been explained. Likewise, it is possible to account for the observation of Raman inactive fundamentals as well as the influence of the forbidden (2)A(u)-(1)A(g) transition on the SERS spectra if the symmetry of the metal-adsorbate complex and the nonplanarity of the (2)A(u) state are taken into account. All the results point out that the CT mechanism is mainly responsible for the SERS features of this molecule studied here. (C) 2000 American Institute of Physics. [S0021-9606(00)70910-X].