EVAPORATION OF COVALENT CLUSTERS - UNIMOLECULAR DECAY OF ENERGIZED SIZE-SELECTED CARBON CLUSTER IONS (CN+, 5 LESS-THAN-OR-EQUAL-TO N LESS-THAN-OR-EQUAL-TO 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 C30+ (factor of 5 to 10). Additionally, low rate constants, relative to the neighbors, are found for C50+, C60+, and C70 +. 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. © 1990 American Institute of Physics.