Photoelectron spectroscopy of small chromium clusters: Observation of even-odd alternations and theoretical interpretation

We report an extensive photoelectron-spectroscopy study of size-selected ${\mathrm{Cr}}_{\mathrm{n}}^{\mathrm{-}}$ (n=2–70) clusters. Even-odd alternations are observed for the small ${\mathrm{Cr}}_{\mathrm{n}}$ clusters and their electron affinities. The spectra of clusters with an even number of atoms show less density of states near the threshold, while those with odd clusters show more complex features. A dimer growth path found in a previous theoretical study is used to interpret the data and accounts well for the even-odd effects. The work provides a key contribution to the understanding of the complicated electronic structure of the Cr clusters.