Enhanced polarizability of aromatic molecules placed in the vicinity of silver clusters

We use a charge-dipole interaction model to study the polarizability of aromatic molecules thatare placed between two silver clusters. In particular we examine the enhancement inpolarizability induced by the clusters at plasmon-like resonant frequencies of the cluster-molecule-cluster system. The model used for these simulations relies on representation of the atoms by both a net electric charge and a dipole. By relating the time variation of the atomic charges to the currents that flow through the bonds of the structures considered, a least-action principle can be formulated that enables the atomic charges and dipoles to be determined. We consider benzene, naphthalene and anthracene for this study, comparing the polarizability of these aromatic molecules when placed in the middle between two Ag₁₂₀ clusters, with their polarizability as isolated molecules. We find that the polarizability of these molecules is enhanced by the clusters, and this increases the electromagnetic coupling between the two clusters. This results in significant red-shifting (by up to 0.8eV) of the lowest energy optical transition in the cluster-molecule-cluster system compared to plasmon-like excitation inthe cluster-cluster system. The resulting resonant polarizability enhancement leads to an electromagnetic enhancement in surface-enhanced Raman scattering of over 10⁶.