Emergence of metallic properties in alkali-rich alkali-halide clusters

We have used photoelectron spectroscopy to study alkali-rich sodium iodide and cesium chloride clusters in order to determine how their composition affects their electronic properties. Spectral features of (Na I)(n)Na-m(-) (m greater than or equal to 2) are remarkably similar to those of isolated metal clusters Na-m(-) for all sizes of n that we investigated. This result suggests that the clusters are comprised of phase separated metallic and ionic components. In contrast, photoelectron spectra of (CsCl)(n)Cs-m(-) clusters are unlike those of pure metal clusters, suggesting that the excess metal atoms are mixed into the ionic portion of the cluster. Such mixing may make it possible for these cesium chloride clusters to exhibit the finite system equivalent of a metal-insulator transition.