Experimental Evidence for the Entropy Effect in Coulombic Cluster Fission

Coulombic fission of warmed triply charged potassium ion clusters ${\mathrm{K}}_n^{3+}$, with $n=54\mathrm{}$–105, is studied by unimolecular dissociation mass spectroscopy. Binary fission is shown to occur predominantly with an asymmetrical character which attenuates as the size of the cluster increases. Such a result cannot be interpreted by energetic considerations unless the entropy contribution of discrete atoms is taken into account.