CAPTURE AND IONIZATION OF ARGON WITHIN LIQUID-HELIUM DROPLETS

Liquid helium droplets of initial mean cluster size, [N], ranging from 600 to 8000 atoms are; doped with argon using the pick-up technique. The doped clusters are ionized by electron impact, and the resulting f ragment ions are monitored as a function of argon pressure in the pick -up volume. Analysis of the pressure dependent ion signals is used to determine (1) the probability for charge transfer from He+ to the Ar a toms within the droplet, and (2) the probability for fragmentation of the Ar-k subclusters upon ionization. The measured charge transfer pro bability from He+ to Ar ranges from 0.05+/-0.02 for clusters of mean o riginal size [N] = 8000 to 0.26+/-0.05 for [N] = 600. Charge transfer to the Ar-k constituent results in the following qualitative trends; a single Ar atom yields HenAr+ ions; Ar-2 mainly yields Ar-2(+), and Ar -3 mainly fragments to yield Ar-2(+). Simulations of the results are p erformed to extract information on how the charge transfer and fragmen tation processes within the ionized droplet dependent on the size of t he helium droplet and the number of argon atoms captured. We use the p ositive-hole resonant-hopping mechanism to determine that the He+ hops 3-4 times prior to localization with either the Ar dopant or another He atom to form He-2(+). This corresponds to a time scale for He-2(+) formation of 60-80 fs. (C) 1998 American Institute of Physics.