On Metal to Insulator Transitions in Bivalent Metal Clusters

We discuss the issue of metal to insulator transitions in bivalent metal clusters, in view of our new photoelectron spectroscopy (PES) studies on Zn$_{n}^{-}$ clusters in the size range of n=3-117. We show that zinc clusters exhibit a distinct transition in their electronic structure characteristics as a function of their size. At small sizes up to $n$=18 the clusters follow the Bloch-Wilson picture of the development of a metal from closed-shell atoms, exhibiting a gradual decrease of the gap between the fully occupied $s $band and the empty $p $band. For large sizes (\textit{n$\ge $}32 ) the band overlap allows the valence electrons to fully delocalize. This leads to an almost perfect free-electron density of states, as is demonstrated by discussing the spectra in the light of standard free-electron models and by comparison to the results obtained on sodium clusters. These results will be compared with the PES of Hg$_{n}^{-}$ and Mg$_{n}^{-}$ clusters.