Evaporation of covalent clusters: Unimolecular decay of energized size-selected carbon cluster ions (C+n, 5≤n≤100)

The unimolecular decay of energized size-selected carbon clusters (C+n, 5≤n≤100) is investigated. The clusters are produced in a laser-generated plasma on the surface of a graphite rod. Directly extracted cations that decay on a μs time scale are probed in a double-focusing, reverse-geometry mass spectrometer. The unimolecular decomposition rates are extracted from metastable fraction measurements. We observe a dramatic discontinuous increase in the decay rate constant as a function of cluster size around mass C+c0 (factor of 5 to 10). Additionally, low rate constants, relative to the neighbors, are found for C+50, C+60, and C+70. The results are rationalized by postulating a phase transition from small ‘‘rigid’’ clusters for n<30 to larger ‘‘molten’’ entities for n>30. In this model local deviations in rate constant reflect the thermodynamic stabilities of the clusters. A further consequence of this model is that ‘‘magic’’ numbers in the mass spectrum originate primarily from the intrinsic stability of the clusters with respect to evaporation and not from a kinetic growth mechanism.