Fission of doubly ionized calcium clusters

Cluster ions, Ca-N(+) and Ca-N(2+) containing up to N = 8 atoms are studied within density functional theory. Ground and first excited states, and ionization energies are reported for all sizes. At zero temperature Ca-3(2+) and Ca-4(2+) are linear, whereas Ca-5(2+) through Ca-7(2+) undergo structural transitions from 3D-configurations into linear ions below 600 K. As a consequence, fission that occurs above 600 K starts from linear configurations. Ca-8(2+) has an hexagonal bipyramidal structure. The preferred fission channels are Ca-N(2+) --> Ca+ + Ca-N-1(+) with fission barriers smaller than the evaporation energy up to Ca-7(2+). However, Ca-8(2+) presents a large fission barrier and would rather evaporate one atom than undergo fission. (C) 2004 Elsevier B.V. All rights reserved.