The Evaporative Ensemble

Abstract

Free and supported particles are generally different, and properties such as energy and structure are best studied in their pure form in free particles to avoid the effects of the interaction with a substrate, unless this interaction is the object of study. A large number of studies have therefore been performed on free clusters, either in molecular beams or in devices such as storage rings or ion traps, which are devices where ions can be stored for extended periods of time. If the particles are produced sufficiently cold and the experiments do not involve a significant energy transfer to the particle, no reactions that change the mass and/or the charge will occur during the experimental time. This means that one can identify the precursor of the products one is measuring in an experiment, which is not a trivial matter for neutral particles given the usually broad mass distributions produced in most particle sources. For these situations one can describe the systems with the microcanonical ensemble, apart from the short times during which the particles are exposed to some external manipulation (laser light, collisions), and possibly exchange of thermal radiation with the surroundings. Or more correctly, by a collection of microcanonical ensembles, very likely with different but individually conserved energies between the scientists’ probing.