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.

The heat capacity of a free cluster has been determined from the temperature dependence of its photofragmentation mass spectrum. The data for the spherical sodium cluster Na 1 N , with N 139, show a maximum at 267 K, which is interpreted as the solid-to-liquid phase transition in this finite system. The melting point lies 104 K, or 28% lower than that of bulk sodium. The latent heat of fusion is reduced by 46%.

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.

The temperature dependence of the optical response of small sodium cluster ions, Na n ϩ (nϭ4, . . . ,16), was measured. At low temperature ͑35 to 100 K͒ individual lines are seen, whose peak positions agree with ab initio calculations if the geometric positions of the atoms in the cluster are taken into account. The peaks broaden slowly with increasing temperature and coalesce, until at high temperature only one, two, or three broad peaks remain, whose positions are for nу7 in good agreement with the nearly free-electron model. Although the detailed shape of the optical spectra changes strongly with temperature, its moments are nearly temperature independent. No pronounced effect due to the solid to liquid phase transition has been observed for these small cluster sizes.

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