Direct Observation of Band-Gap Closure in Mercury Clusters

We have measured the photoelectron spectra of mass-selected negatively charged mercury clusters Hgₙ^- in the size range n0ex0ex=0ex0ex3--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 n0ex0ex=0ex0ex400±30, a considerably larger value than previously reported (n0ex0ex=0ex0ex80--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.