Evolution of Electronic Energy Levels in Krypton Clusters from the Atom to the Solid

We deduce the evolution of electronic excitations in Kr clusters ($N=2-3000\mathrm{}$) from fluorescence excitation spectroscopy with synchrotron radiation. In small clusters ($N\le 30$), a broad absorption band appears, slightly red shifted compared with the atomic $5s{\left(\frac{3}{2}\right)}_1$ resonance line. Intermediate-size clusters ($50<N\le 200$) show absorption profiles which are mainly correlated with the surface excitons of the solid. The bulk excitons are pronounced only in large clusters ($N\ge 200$). Absorption processes at the surface and inside the cluster correlate well to the surface-to-volume ratio of the clusters.