Charge localization and fragmentation dynamics of ionized helium clusters

The dynamics of He-n(+), n = 3 - 13, clusters formed by electron impact ionization of the neutral is studied theoretically using mixed quantum/classical dynamics by both mean-field and surface hopping methods. Potential energy surfaces and nonadiabatic couplings among them are determined from a semiempirical, minimal basis DIM Hamiltonian. The dynamics of hole hopping, hole localization, and cluster fragmentation are described through trajectory data. He-3(+) clusters, with initial conditions given by the zero-energy quantum distribution of nuclear coordinates, dissociate through two-channels, He+He+He+ and He+He-2(+) with relative yields of 20% and 80%. The motif of hole localization on a pair of atoms, and subsequent dissociation of the initial pair with hole hop to a new pair is observed in trimers, and repeats in larger clusters. In the larger clusters, hole hopping among He-2 pairs provides an additional, less important mechanism of charge migration. The coupled electronic-nuclear dynamics of triatomic units describes the mechanism of energy loss, by transfer of vibrational to translational energy. This leads to ejection of energetic neutral atoms as well as the ejection of He-2(+) prior to evaporative cooling of the cluster. He-2(+) is the exclusive charged unit produced in the fragmentation of He-13(+) clusters. In bulk He the same dynamics should lead to fast vibrational relaxation t<10 ps and formation of He-3(+) as the positive ion core. (C) 1998 American Institute of Physics.