Caloric curve for the liquid-to-gas transition of the H25+ hydrogen cluster ion

Abstract High energy collisions (60 keV/amu) of the H 25 + hydrogen cluster ions with an helium target have been completely analyzed on an event-by-event basis in a recently developed multi-coincidence experiment. By selecting specific decay reactions we can start after the energizing 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 (plateau region) of a first order phase transition in a finite system, in the present case signaling the transition from a bound cluster to the free gas phase type situation.