DFT study of adsorption site effect on surface-enhanced Raman scattering of neutral and charged pyridine-Ag4 complexes

Density functional theory (DFT) and time-dependent DFT (TDDFT) methods have been used to investigate the adsorption site effect of Raman scattering for neutral and charged pyridine-Ag-4 complexes. The Calculated results show that the SERS spectra are strongly dependent on adsorption site and the configuration of new complexes. The normal Raman spectra of neutral and charged pyridine-Ag-4 complexes are similar with that of isolated pyridine but with an enhancement factor below 10 times. This enhancement is ascribed to ground state chemical enhancement. The pre-surface-enhanced Raman scattering (SERS) spectra were calculated at 1256 rim, 769 nm and 744.3 nm, which are nearly resonant with the charge transfer excited states S-2 for neutral and charged pyridine-Ag-4 complexes, respectively. We obtain the enhancement factor about 104 to 105 in pre-SERS spectra which is mainly Caused by charge transfer resonance Raman enhancement. The three-dimensional cube representation is also applied to describe the photoinduced CT, which are considered as direct evidence of chemical enhancement, between pyridine and two isomers of Ag-4 clusters. (C) 2009 Elsevier B.V. All rights reserved.