Indexed on: 16 Apr '15Published on: 16 Apr '15Published in: Theoretical Chemistry Accounts
This work presents the application of a recent decomposition scheme of the Raman tensor into molecule and surface contributions to the study of the static and resonance Raman spectra of pyridine adsorbed on a Ag20 cluster, a typical probe for the theoretical study of surface-enhanced Raman scattering (SERS) spectra. The results obtained show that both the chemical and electromagnetic enhancements observed are related to changes on the polarizability and polarizability derivatives of the pyridine molecule. No significant contributions from the surface and from vibrational intermolecular coupling are found. Since similar incident lights produce remarkably different SERS spectra, the effect of excitation wavelength on the spectra of the PY–Ag20 complexes is also scrutinized. From the computed Raman excitation profiles and from the analysis of the electron density changes upon electronic transitions, it is established that the differences found can be related to the amount of electron density transferred from the silver cluster to pyridine upon excitation and to the distance between both units. These findings suggest that a proper knowledge of the effect of the excitation wavelength is necessary for obtaining a reliable theoretical interpretation of surface-enhanced Raman spectra.