 
Summary: Charge localization and fragmentation dynamics of ionized helium clusters
M. Ovchinnikov, B. L. Grigorenko,a)
K. C. Janda, and V. A. Apkarian
Department of Chemistry, University of California, Irvine, California 926972025
Received 9 February 1998; accepted 6 March 1998
The dynamics of Hen , n 313, clusters formed by electron impact ionization of the neutral is
studied theoretically using mixed quantum/classical dynamics by both meanfield 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. He3 clusters, with
initial conditions given by the zeroenergy quantum distribution of nuclear coordinates, dissociate
through twochannels, He He He and He He2 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 He2 pairs provides an additional, less important mechanism of charge migration.
The coupled electronicnuclear 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 He2 prior to evaporative cooling of the cluster. He2 is the exclusive charged
unit produced in the fragmentation of He13 clusters. In bulk He the same dynamics should lead to
fast vibrational relaxation t 10 ps and formation of He3 as the positive ion core. © 1998
