Spectroscopic properties of Na3+ in bulk and on the surface of Ar droplets

We present a theoretical study of Na3+ embedded in- and on the surface of a rare-gas droplet. We describe the active electrons of the cluster using time-dependent density functional theory, while the surrounding Ar atoms are treated by classical molecular dynamics of polarizable atoms. Like for neutral clusters, we found that the surrounding material modifies noticeably the optical properties of Na3+ compared to the free cluster. Ar polarization and core repulsion act in opposite directions, so that the actual line shift depends significantly on the trapping site. We show that the long range monopole-dipole coupling between a charged cluster and a polarizable matrix makes Na3+ more likely to be embedded inside the Ar matrix, while small neutral Na clusters are more likely to be trapped at the surface.