Collision induced fragmentation of small ionic sodium clusters: Competition between electronic and impulsive mechanisms

Collision induced fragmentation of small Na-n(+) (n = 3 - 9) clusters with He atoms is investigated in the 100 eV center-of-mass collision energy range. The experiment is based on the determination of the velocity vectors of the fragments using a multicoincidence technique. The relative populations of the various fragmentation pathways are determined. Fragmentation mechanisms are discussed in detail. The most important pathways are primarily populated via momentum transfer in elastic binary collisions between the He atom and a Na+ core. Direct release of fast Na atoms is observed at variance with what is usually assumed at eV energies. However most of the fragmentation involves multistep dynamics with energy redistribution inside the cluster via Na-Na collisions. In contrast, production of Na+ fragments comes dominantly from electronic transitions towards repulsive potential energy surfaces of the cluster. The role of electron pairing is emphasized. (C) 1999 American Institute of Physics. [S0021-9606(99)01022-3].