Coulombic fission and evaporation of antimony cluster ions

Abstract

The preferential dissociation channels of singly and doubly charged antimony clusters have been determined from the unimolecular dissociation of energy rich cluster ions, using an ion stopping technique. It is found that singly charged Sb+n clusters with 5≤n≤80 dissociate by loss of neutral molecules. Neutral dimer loss is observed for n=5, 6, 7 whereas for n≥8, Sb+n loses Sb4. The fragmentation of doubly charged Sb++n has been investigated above the critical size n++c=24 from which doubly charged clusters are detectable in mass spectra. On the time scale of the experiment, which is 1 μs≤t≤100 μs with respect to photoionization, the delayed Coulombic fission into two singly charged clusters competes with the evaporation of Sb4. It is shown that for the smaller Sb++n clusters with 26≤n≤36 the fission to two singly charged clusters is of relatively asymmetrical character, leading to the detachment of five and seven atom cationic fragments. In larger clusters n≥40 the fission is of more symmetrical character and the difference in the fission products is less than 30%. Such a behavior totally deviates from the drop model predictions.