Illustrative direct ab initio calculations of surface Raman spectra

Illustrative ab initio calculations of Raman spectra of pyridine (Pyr) and pyridine in metal-molecule complexes with silver (Ag+-Pyr, Ag5+-Pyr) have been undertaken. The Raman spectra were computed numerically in the standard Placzek approximation with the additional feature of a static external electric field, F, perturbation to the system Hamiltonian. F was allowed to change the system equilibrium geometry and symmetry as well as the polarisability, resulting in shifted vibration frequencies and changed Raman intensities. All calculations were done on a Hartree-Fock (HF) level using the 6-31+G* or the Sadlej p-VTZ basis sets for Pyr, and the Stevens Krauss (SBKJC) VDZ ECP basis set for Ag. Raman enhancement factors over 5 orders of magnitude were obtained. The enhancement was analyzed as being due to polarization and charge transfer induced by the metal-molecule contact in the absence of F and in the presence of a strong F as due to increased polarisability derivatives along the surface normal. The use of the present method to qualitatively and quantitatively predict surface Raman spectra is discussed.