Experimental study of the transition from van der Waals, over covalent to metallic bonding in mercury clusters

Abstract

The following properties have been measured for mercury clusters: (1) ionisation potentials of Hg_n by electron-impact ionisation, (2) dissociation energies of Hg⁺_n, and (3) mass spectra for negatively charged mercury cluster ions (n 3). Cohesive energies for neutral and ionised Hg clusters have been calculated from the data. The transitions in chemical binding are discussed. For small clusters Hg_n is van der Waals bound (n⩽ 13), the binding changes to covalent for 30 ⩽n⩽ 70, and then to metallic (n 100). A sudden transition from covalent to metallic bonding is observed. It is discussed whether this can be considered as being analogous to a Mott transition for a finite system. The experimentally observed transitions in chemical bonding are much more pronounced than those calculated in a tight-binding calculation. This points to strong correlation effects in Hg clusters.