DFT study of chemical mechanism of pre-SERS spectra in Pyrazine–metal complex and metal–Pyrazine–metal junction

The chemical mechanism of Normal Raman Scattering (NRS) and pre-surface enhanced Raman scattering (pre-SERS) spectra for Pyrazine–Ag2 complex, Ag2–Pyrazine–Ag2 junction and Ag2–Pyrazine–Au2 junction were investigated with density functional theory (DFT) and charge difference densities (CDDs) for the first time. The NRS intensities of the above three structures enhanced obviously relative to isolated Pyrazine and the enhancement mechanism was confirmed to be static chemical enhancement. The pre-SERS intensities of the above three structures enhanced evidently compared to corresponding NRS intensities, and the enhancement mechanism was confirmed to charge transfer (CT) resonance Raman enhancement. The largest enhanced orders of NRS and pre-SERS intensities among the three structures were up to 103 and 105, respectively. Compared the intensity of pre-SERS with corresponding intensity of NRS spectra, the enhancement effect of Pyrazine–Ag2 complex was larger than the others. Intramolecular and intermolecular CT on resonant electronic transition were described by CDDs.