Studies of the Structure, Binding Properties and Defects in Alkali-Halide Microclusters.

We have studied molecular clusters of alkali-halide materials using both an experimental molecular beam apparatus and numerical molecular dynamics simulations. We have explored cluster ions produced directly by laser vaporization of a solid sample both with and without growth after the vaporization. The laser vaporization source we used produces cluster mass spectra with variations that measure the stability against high temperature decay. The mass spectra of fully ionic clusters shows that clusters with cuboid geometries are particularly stable and that cuboid clusters with terrace steps are also particularly stable. Molecular dynamics calculations were performed to explore the decay rate as a function of cluster size for these clusters. Further we have observed a number of types of alkali-halide clusters not previously observed in mass spectra. We have seen clusters of both charge states with either excess electrons, excess chlorine atoms and two excess electrons beyond the ionic clusters. These clusters are thought to have the same structure as color centers in the bulk. Further experiments were done on mass selected cluster ions to determine the binding energy of electrons in the excess electron clusters. Numerical simulations using path integral molecular dynamics were done to characterize the electron localization and binding energies in these clusters.