Direct Experimental Evidence for a Negative Heat Capacity in the Liquid-to-Gas Phase Transition in Hydrogen Cluster Ions: Backbending of the Caloric Curve

Abstract

By selecting specific decay reactions in high-energy collisions (60 keV/amu) of hydrogen cluster ions with a helium target (utilizing event-by-event data of a recently developed multicoincidence experiment) and by deriving corresponding temperatures for these microcanonical cluster ensembles (analyzing respective fragment distributions), we are able to construct caloric curves for H+3(H2)(m) cluster ions (6<or=m<or=14). All individual curves and the mean of these curves show a backbending in the plateau region, thus constituting direct evidence for a negative microcanonical heat capacity in the liquid-to-gas transition of these finite systems.