Caloric curve across the liquid-to-gas change for sodium clusters.

The caloric curve for Na(139)(+) has been measured from 100 K up to the temperature where the clusters are boiling hot and spontaneously emit atoms. In this limit the clusters form an evaporative ensemble, the temperature and energy of which have been determined. As the caloric curve of an atomic gas with a finite number of atoms is known, one can construct the caloric curve for this finite system below and above the boiling point. A conjecture is made on how to link the evaporative ensemble temperature of the free cluster in vacuum to the boiling temperature of a finite system at a given pressure. This allows one to determine the enthalpy of vaporization at the phase transition of the finite system.

Negative heat capacity for a cluster of 147 sodium atoms.

There exists a surprising theoretical prediction for a small system: its microcanonical heat capacity can become negative. An increase of energy can-under certain conditions-lead to a lower temperature. Here we present experimental evidence that a cluster containing exactly 147 sodium atoms does indeed have a negative microcanonical heat capacity near its solid to liquid transition.