Collision induced cluster fragmentation: From fragment size distributions to the caloric curve

Abstract

We report on a cluster fragmentation study involving collisions of high-energy (60 keV/amu) H 3 + (H 2 ) m hydrogen cluster ions (m=9,11) with atomic helium or fullerenes. The experimental characterisation of the cluster fragmentation not only by the average fragment size distribution but also by a statistical analysis of the fragmentation events has become possible owing to a recently developed multi-coincidence technique in which all the fragments of all collisions occurring in the experiment are mass analysed on an event-by-event basis. By selecting specific decay reactions we can start after the energising collision with a microcanonical cluster ion ensemble of fixed excitation energy. From the respective fragment distributions for these selected decay reactions we derive corresponding temperatures of the decaying cluster ions. The relation between this temperature and the excitation energy (caloric curve) exhibits the typical prerequisites of a first order phase transition in a finite system, in the present case signalling the transition from a bound cluster type situation to the free gas phase.