Understanding complex surface-enhanced Raman scattering, using quantum chemical calculations

Several key issues concerning surface-enhanced Raman scattering (SERS) are discussed, especially the surface orientation of the adsorbate, the enhancement mechanism relevant in a particular experiment, and the dependence of SERS frequencies and intensities on the electrode potential. It is shown that ab initio calculations of energies, geometries, and vibrational frequencies are essential in order to understand the complex behavior displayed by SERS spectra, particularly calculations dealing with radical anions of adsorbates. The main conclusion is the confirmation of the presence of resonant charge transfer processes in SERS of aromatic molecules. Such resonant mechanism involves the transient formation of the radical anion of adsorbates such as pyrazine, pyridine, and derivatives. (c) 2005 Wiley Periodicals, Inc.