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

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