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.
Highlights
Phase transitions are universal properties of matter in interaction
It has been proposed that phase transitions in finite systems within the microcanonical ensemble [1,6] may be detected by a characteristic functional relationship between the temperature and the excitation energy, the so-called caloric curve, i.e., a first order phase transition should correspond to a negative branch for the heat capacity [7,8,9]
Mass selected hydrogen cluster ions with an energy of 60 keV/amu are prepared in a high-energy cluster ion beam facility [12] consisting of a cryogenic cluster jet expansion source combined with a high performance electron ionizer and a two step ion accelerator
Summary
Phase transitions are universal properties of matter in interaction. They have been extensively studied in the thermodynamical limit of infinite system. After the energizing collisions between 60 keV/amu ions with a helium target we select a microcanonical ensemble of cluster ions with a given energy (by selecting specific decay reactions from a host of possible decay channels) and determine from the (partial) fragment size distribution the corresponding temperature of the ensemble of decaying cluster ions This experiment involves the complete event-by-event analysis of 11,685 collisions using a recently developed [12,13] multi-coincidence technique for the simultaneous detection of the correlated, ionized and neutral, collision fragments allowing us to obtain an experimental caloric curve for the transition from a bound cluster to the gas phase
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