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 rang e. 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 mechani sms are discussed in detail. The most important pathways are primarily popu lated via momentum transfer in elastic binary collisions between the He ato m and a Na+ core. Direct release of fast Na atoms is observed at variance w ith what is usually assumed at eV energies. However most of the fragmentati on involves multistep dynamics with energy redistribution inside the cluste r via Na-Na collisions. In contrast, production of Na+ fragments comes domi nantly from electronic transitions towards repulsive potential energy surfa ces of the cluster. The role of electron pairing is emphasized. (C) 1999 Am erican Institute of Physics. [S0021-9606(99)01022-3].