Dissociation dynamics of vibrationally excited van der Waals clusters: I₂XY→I₂+X+Y (X, Y=He, Ne)

The dynamics of sequential dissociation processes of the type XI ₂(v)Y→X+I₂(v′)Y→X+Y+I₂(v″ )(X, Y=Ne, He) are studied using classical trajectory calculations and a recently presented classical version of the time-dependent self-consistent field (TDSCF) method. The results obtained indicate the presence of significant dynamical correlation effects of the rare-gas atoms on each other despite the negligible direct interaction between them; this is in qualitative agreement with experimental findings. Good agreement is found for the rate constants and the variation with rare gas as well as the branching ratios (NeI ₂He→NeI₂+He vs→I₂He+Ne) calculated from TDSCF and from classical trajectories. Both classical trajectories and TDSCF show an essentially impulsive dissociation mechanism, in which dissociation typically follows a considerable number of vibrations, and is due to a relatively rare internal hard collision between an I atom and the rare gas. As in the three-body I₂X case, this mechanism differs from that in the RRKM strong coupling model. Energy- and momentum-gap relations, based on the weak-coupling picture, are found to be relatively successful but fail to describe the dynamics quantitatively.