Character of tightly bound excitons in small argon clusters: Insights from size-dependent energy shifts

The structure of the first electronically excited states of small ArM̄ clusters (M̄<~100) embedded in large NeN̄ (N̄=7500) clusters is investigated using fluorescence excitation spectroscopy. In the energy range of the characteristic absorption of Ar clusters (11.5--12.9 eV), surface excitons of Ar clusters embedded in Ne disappear, while additional absorption bands appear. They are assigned to excitons at the interface between the Ar cluster and the Ne host cluster. The observed energy shift of all absorption bands is proportional to the logarithm of the cluster radius. This can be understood in the Frenkel-exciton model, taking the resonant excitation transfer into account.