COULOMB BARRIERS IN THE DISSOCIATION OF DOUBLY CHARGED CLUSTERS

The barrier height for the most asymmetric fission decay of doubly charged sodium clusters (Na(N)2+) into singly ionized fragments has been computed with use of density-functional theory and the jellium model. We have found that the barrier is sizable for large or intermediate-size clusters, but vanishes for N less-than-or-equal-to 9. We have also computed the energy DELTA-H(e) needed to evaporate a neutral monomer from Na(N)2+. For N less-than-or-equal-to 40, the barrier height is smaller than DELTA-H(e), and emission of a Na+ ion is the preferred decay channel of hot Na(N)2+ clusters. On the other hand, the barrier height is larger than DELTA-H(e) for N >40 and, in this case, monomer evaporation becomes competitive. The critical cluster size, N(c) = 40, for the transition from one decay mode to the other is in reasonable agreement with the experimental result. Our calculations suggest that the mechanism for neutral-monomer evaporation is different from the one currently assumed.