Spectral Characteristics of Chemical Enhancement on SERS of Benzene-like Derivatives: Franck-Condon and Herzberg-Teller Contributions

A systematically theoretical investigation has been performed to study the dynamic chemical enhancement of surface-enhanced Raman spectroscopy (SERS) of pyridine, pyrimidine, 2-mercaptopyridine, and 4-mercaptopyridine absorbed on a silver cluster of Ag₂₀. The influences of different structural configurations (V and S), different intermolecular charge-transfer (CT) excited states, and the different approximations [Franck-Condon (FC) or FC + Herzberg-Teller (FCHT) approximations] to spectral cross sections have been examined. It is found that the photoexcitation can easily produce the intermolecular CT excited states, leading to the absorption maxima red-shift, and their intensities decrease compared with that of Ag₂₀. Furthermore, we observe that the absolute Raman intensities are sensitive to the systems' structural configurations, exchange-correlation functionals, CT excited states, and the FC/FCHT approximations as well. However, the relative Raman intensities and dominant vibrational structures of CT resonance RS (RRS) are mainly determined by the adsorbates. The modes which can have a larger enhancement in all CT RRS are those related to ring stretch and ring breathing. The ring-stretching mode at around 1600 cm^-1 in four molecule-Ag₂₀ systems is evidently enhanced compared to that of bare molecules which can be considered as a hint of the presence of the CT resonance enhancement. Additionally, we observe that HT effects dominate the resonance enhancement and it could explain the coupling between the plasmonic and chemical enhancement mechanisms, but the FCHT approximation does not significantly change the relative RRS intensities obtained through the FC approximation.