Direct observation of band-gap closure in mercury clusters

We have measured the photoelectron spectra of mass-selected negatively charged mercury clusters Hg-_n in the size range n = 3–250. The spectra are characterized by gaps which shrink with increasing n. These gaps represent the s − p excitation band gaps of the corresponding neutral clusters. Extrapolation to higher cluster size indicates band gap closure at the size range of n = 400 ± 30, a considerably larger value than previously reported (n = 80–100) . This new evaluation indicates that previous experimental criteria for the band closure are not appropriate and calls for a refined theoretical formulation of the electronic structure of mercury clusters.