Structure and dynamics of alkali-metal clusters and fission of highly charged clusters

Abstract

A dynamical optimization of the minimum energy cluster structures of Na, K, Rb, and Cs clusters was performed using a many-body potential based on local density calculations. The energetics and vibrational analysis 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 charged clusters undergo fission depends strongly on the cluster temperature. Three phases in the temperature-size-phase plane are identified corresponding to unstable, metastable, and stable clusters. These regions are bound by the spontaneous size at zero temperature and the critical size at the critical temperature. The cluster critical size exhibits 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 limitations of the liquid drop model are indicated in light of the dynamical findings.