Collision induced fragmentation of small ionic sodium clusters. II. Three-body fragmentation

Multiple fragmentation of small Na-n(+) clusters (n=3-5) induced by collisi on with He atoms is investigated in the 200 eV collision energy range. The Na-n(+)-->Na-n-2(+)+Na+Na channels are studied using a multicoincidence tec hnique allowing for the determination of the velocity vectors of the three fragments. The relative contributions of the two basic mechanisms, namely t he electronic excitation and the momentum transfer in elastic binary collis ions between the He atom and a Na core are estimated. For Na-4(+) and Na-5( +) clusters the momentum transfer mechanism appears as a two-step process: The ejection of a fast Na atom in a binary Na-He collision followed by the evaporation of an additional slow Na atom. However the angular distribution of the slow Na atom is not isotropic showing that a partial memory of the initial binary collision appears in the evaporation process. (C) 2000 Ameri can Institute of Physics. [S0021-9606(00)00127-6].