Computational Materials Science Center: PUBLICATIONS

Molecular dynamics simulations of Na, K, Rb and Cs clusters were performed using a newly developed many-body potential based on local density calculations within the second moment approximation. The energetics of neutral clusters in the size range 13 < N < 310 are calculated, including the free energy as a function of cluster size. The fission process due to Coulomb forces of 2+, 3+, and 4+ charged 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 size regions are identified corresponding to unstable, metastable and stable clusters in the temperature-size plane. These regions are bound by the spontaneous size at T=0 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.

Click on Image to Enlarge Time evolution of a cluster with 55 sodium atoms and a charge Q=4 undergoing symmetric fission at a temperature of 550 K. top left at time=0 bottom left at t=350 ps bottom right at t=400 ps top right at t= 450 ps The colors were given after fission occurred to differentiate the atoms in one of the fission fragments from those in the other fragment. As is apparent, the atoms were initially randomly distributed before the ionization event, and gradually self organize during the fission process. A strong ellipsoidal deformation takes place during fission.