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 ${\mathrm{Na}}_N^{+}$, with $N=139\mathrm{}$, 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%.

• Received 13 March 1997

DOI:https://doi.org/10.1103/PhysRevLett.79.99