Size dependence of the gradual transition to metallic properties in isolated mercury clusters

Single-photon ionization thresholds are reported for mercury clusters with up to 70 atoms prepared in a supersonic beam. The size dependence of the ionization potentials provides the first direct experimental evidence that a heretofore undetected size-dependent gradual transition from van der Waals type to metallic properties occurs for mercury clusters in the size range between 20 to approximately 70 atoms.