Size-dependence of the enhanced Raman scattering of pyridine adsorbed on Ag_n (n = 2-8, 20) clusters

We present a time-dependent density functional theory (TDDFT) study of the size-dependence of the absorption and Raman scattering properties of pyridine-interacting with small silver nanoclusters Ag_n (n = 2-8, 20). By employing a recently developed short-time approximation for the Raman scattering cross section we simulate both the normal and the "surface"-enhanced Raman spectra. The absorption spectra of the small silver clusters are studied both in the gas phase and embedded in rare gas matrices. We find that both the absorption and Raman properties depend strongly on cluster size and adsorption site. The normal Raman spectra of Ag _n-pyridine complexes resemble that of isolated pyridine, with enhancements which increase as cluster size increases. In contrast to this, both the enhancement and the appearance of the "surface"-enhanced Raman spectrum show a very strong dependence on cluster size. The total enhancements for the complexes are between 10³ and 10⁴ and quite surprisingly the strongest enhancement is found for the Ag₂-Py complex. However, the enhancement trends can be correlated with the distance of the molecule to the center of the metal cluster and with the resonance polarizability in a way that is suggestive of electromagnetic enhancements, so the enhancement mechanism for these small clusters is similar to what is found for larger particles.