Ionization and fragmentation of Bi microclusters by electron impact

Results are presented for an electron-impact ionization study of Bi microclusters ranging in size from one to thirty-eight atoms. By comparing results from two different cluster source conditions, we identify a clear onset for fragmentation in the vicinity of 10.5 eV. This is below the threshold for double ionization, yet well above the first ionization thresholds. The latter thresholds have been found to be almost universally double and are quite sharp, suggesting unique structures for each cluster size. The first threshold energies vary with size in a manner consistent with the classical metal-sphere model.