STRUCTURE AND DYNAMICS OF ALKALI-METAL CLUSTERS AND FISSION OF HIGHLY-CHARGED CLUSTERS

A dynamical optimization of the minimum energy cluster structures of N a, K, Rb, and Cs clusters was performed using a many-body potential ba sed on local density calculations. The energetics and vibrational anal ysis of the neutral clusters in the size range 8<N<310 were calculated , including the free energy as a function of the cluster size and the melting temperature. The fission process due to Coulomb forces of 2+, 3+, and 4+ charged alkali-metal clusters was studied extensively using molecular dynamics. We show that the cluster size at which multiply c harged clusters undergo fission depends strongly on the cluster temper ature. Three phases in the temperature-size-phase plane are identified corresponding to unstable, metastable, and stable clusters. These reg ions are bound by the spontaneous size at zero temperature and the cri tical size at the critical temperature. The duster critical size exhib its a power law dependence on the total charge, which is in excellent agreement with experiments. The energy barriers that the clusters need to undergo fission are reported as a function of cluster size. The li mitations of the liquid drop model are indicated in light of the dynam ical findings.