Ionization and fragmentation of Bi microclusters by electron impact

Results are presented for an electron impact study of ionization and fragmentation effects in Bi microclusters ranging in size from 1 to 63 atoms. Using results from both high-vacuum and gas condensation cluster sources, energy thresholds for the fragmentation of clusters larger than 20 atoms or so are identified and found to lie 5±1 eV above the corresponding ionization thresholds. The experimental distribution of net changes in intensity from fragmentation, for clusters from 3 to 63 atoms, has been fitted with a simple model in which it was necessary to assume (a) a strong peak in the fragmentation probability in the vicinity of ∼30 atoms and (b) a probability of ∼80% that the minor fragment retains the positive charge, contrary to expectations based on final energies. Ionization potentials are given for clusters up to n=38 atoms. These are found to show rough agreement with the metallic sphere model for n>5 atoms, i.e., behavior intermediate between that of simple metals and semiconductors. The great majority of clusters studied also exhibit second thresholds in their ionization curves, from 1.2 to 3.0 eV above the first onset. The interval between thresholds becomes nearly constant for n>10 atoms. We tentatively attribute this effect to structure in the filled electron states.