The thermodynamics of clusters in a carrier gas

One of the fascinating aspects of physics is that one sometimes encounters systems that cannot be characterised by established descriptions. In thermodynamics some systems cannot be fit neatly in one of the three common aggregation states: gas, liquid and solid. Well known examples include glasses and liquid crystals, which have properties of both solids and liquids. Here, we discuss a gas of metal clusters, a system that is both solid and gaseous at the same time. Using a cluster beam experiment, it is shown how the equipartition theorem, which assumes energy is shared equally among all degrees of freedom, looses its validity if the particles in an expanding gas are significantly larger than a few atoms. This challenges the concept of temperature itself, requiring multiple temperatures to describe the different amounts of energy stored in translational, rotational and vibrational degrees of freedom.status: publishe