High-resolution ion mobility measurements for silicon cluster anions and cations

High-resolution ion mobility measurements have been performed for silicon cluster anions and cations, Si-n(-) and Si-n(+), n=6-55. New isomers have been resolved for every cluster size larger than Si-20. The results for the anions and the cations have the same global features. However, changing the charge often causes a shift in the isomer distribution, or causes new isomers to emerge. For example, the transition from prolate geometries to more-spherical ones is shifted to larger cluster sizes for the anions than for the cations. The mobilities of the anions are systematically smaller than those of the cations, presumably because of differences in the exterior electron densities. (C) 1999 American Institute of Physics. [S0021-9606(99)01914-5].