Charged cores in ionized 4He clusters. III A quantum modelling for the collisional relaxation dynamics.

When a pure He-4 droplet is ionized by electron impact, the most abundant fragment detected in mass spectra after ionization is He-2(+). All the mo dels that have been proposed thus far to explain the experimental evidence therefore involve the formation of the He-2(+), molecular ion. The understanding of the interactions between this ion and the surrounding He atoms in the cluster and of their dynamical behavior during cluster break-up is an important e lement for the modeling of the cluster evolution after the ionization event. In previous works [1, 2] we have computed and described the Potential Energy S urface (PES) of the electronic ground state for the He-3(+) system that provides the required forces between He-2(+) and He. After ionization He-2(+) is pr esumably formed by association of an He+ and any of the neutral atoms in the cluster via a 3-body collision process. The ensuing vibrational quenching of t he "hot" molecular ion may release the energy necessary to evaporate the entire droplet, or most of it, and give the fragmentation patterns detected by exp eriments. We present here a model quantum dynamics that generates vibrational deexcitation cross-sections and the corresponding rate coefficients for the c ollision of He-2(+) with He. A timescale of the cluster evaporation due to vibrational relaxation is estimated and the present findings are compared with e arlier studies on the same system.