TDDFT studies of absorption and SERS spectra of pyridine interacting with Au20

We present time dependent density functional theory (TDDFT) calculations for a tetrahedral Au20 complex interacting with pyridine for the purpose of modeling absorption and surface enhanced Raman scattering, with emphasis on chemical and electrodynamic enhancement effects. These calculations are done using the ADF code with the BP86 functional, the zeroth-order regular approximation and with the resonant electronic response modeled using a short time approximation expression for the perturbed density matrix, with a damping factor that is empirically chosen. The absorption spectrum of bare Au20 shows strong intraband (sp-sp) and interband (sp-d) coupling with a low-energy peak at 2.89 eV that is mostly intraband and other peaks at 3.94 and 4.70 eV that have mixed intra- and interband character. SERS spectra are calculated for pyridine/Au20 for both vertex (V) and surface (S) configurations at their respective lowest energy absorption maxima (near 2.89 eV), and we find that the V configuration has higher intensities that correspond to SERS enhancements of 10(3)-10(4), whereas S has an enhancement of 10(2)-10(3). These enhancement values are significantly lower than the analogous results for pyridine/Ag20 primarily because of reduced oscillator strength associated with the intraband transition in Au20. Decomposition of the pyridine/Au20 enhancement factor into chemical and electromagnetic contributions (through an analysis of the static SERS intensities) shows enhanced chemical enhancements compared to Ag20 but reduced electromagnetic enhancements.