Electron binding and stability of excess-electron alkali halide clusters: Localization and surface states

Metal-excess alkali halide clusters have been investigated experimentally using a cluster-beam source comprised of laser ablation of alkali metal and reaction of helium-seeded halogen gas. Metal-excess neutrals and charged clusters, MnXm(+) where M=Na or Li and X=F, Cl, or Br, have been produced, with the neutral series, MnXn-1, displaying a wide range of excess-electron-dominated behavior. An extensive investigation of the abundances and electronic properties of NanFn-1 (n<100) is described. A simple quantitative model of electron localization is proposed to account for the observed patterns of abundances and electron binding energies. The model correctly predicts that (i) for the singular values of n that correspond to a filled cubic lattice, the excess electron is very weakly bound, and that (ii) clusters that can form a cubic lattice with a single anion vacancy have the electron localized in the vacancy, stabilizing the neutral cluster against atom loss.