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–Ag4 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–Ag4 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 nm, 769 nm and 744.3 nm, which are nearly resonant with the charge transfer excited states S2 for neutral and charged pyridine–Ag4 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 Ag4 clusters.