We have measured the photoelectron spectra of mass-selected negatively charged mercury clusters ${\mathrm{Hg}}_n^{-}$ in the size range $n=3\mathrm{}–250$. 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 $n=400\mathrm{}\pm 30$, a considerably larger value than previously reported $(n=80\mathrm{}–100)$. 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.

• Received 5 June 1998

DOI:https://doi.org/10.1103/PhysRevLett.81.3836