CHARGE LOCALIZATION AND FRAGMENTATION DYNAMICS OF IONIZED HELIUM CLUSTERS

The dynamics of He-n(+), n = 3 - 13, clusters formed by electron impac t ionization of the neutral is studied theoretically using mixed quant um/classical dynamics by both mean-field and surface hopping methods. Potential energy surfaces and nonadiabatic couplings among them are de termined from a semiempirical, minimal basis DIM Hamiltonian. The dyna mics of hole hopping, hole localization, and cluster fragmentation are described through trajectory data. He-3(+) clusters, with initial con ditions given by the zero-energy quantum distribution of nuclear coord inates, 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 p air of atoms, and subsequent dissociation of the initial pair with hol e hop to a new pair is observed in trimers, and repeats in larger clus ters. In the larger clusters, hole hopping among He-2 pairs provides a n additional, less important mechanism of charge migration. The couple d electronic-nuclear dynamics of triatomic units describes the mechani sm of energy loss, by transfer of vibrational to translational energy. This leads to ejection of energetic neutral atoms as well as the ejec tion of He-2(+) prior to evaporative cooling of the cluster. He-2(+) i s the exclusive charged unit produced in the fragmentation of He-13(+) clusters. In bulk He the same dynamics should lead to fast vibrationa l relaxation t<10 ps and formation of He-3(+) as the positive ion core . (C) 1998 American Institute of Physics.