Observation of cluster-specific excitations inXeNclusters

The absorption spectra of ${\mathrm{Xe}}_{\mathit{N}}$ clusters (N=50–500) exhibit extra absorption bands not seen in condensed Xe gas. They show up if the cluster radius is comparable to the radius of the electronic excitation. From the sharpness of the bands and from the energy-level positions it is concluded that these excitations have a character which combines features seen in molecular Rydberg states and excitons of the solid. This interpretation is supported by simple model calculations. For small clusters (N≤150) the number of extra absorption bands reflects the shell structure of the clusters.