NONEQUILIBRIUM INTRAMOLECULAR TRANSPORT IN ISOMERIZATION DYNAMICS OF AR-7-LIKE MOLECULES IN LIQUID-LIKE PHASE

In isomerization dynamics of Ar-7-like molecules of high energy, which is in the so-called liquidlike phase, a peculiar characteristic has b een observed [K. Takatsuka and C. Seko, J. Chem. Phys. 105, 10356 (199 6)], that is, the occurrence of a given geometric isomerization in a s hort Lifetime is less frequent than expected by an exponential distrib ution based on the mixing in dynamics. This behavior is exactly the re verse to those observed in the simpler system such as dissociation rea ction of H-3(+) [M. Berblinger and C. Schlier, J. Chem. Phys. 101, 475 0 (1994)], in which many of the so-called direct paths are ejected bef ore the mixing takes effect and thereby the short lifetime isomerizati on (or dissociation) occurs more frequently than the exponential distr ibution. The former fact implies that the classical trajectories take somewhat longer time (induction time) to find their ways out to the ot her isomers in phase space, and therefore it can be a prototype of the so-called slow dynamics that is frequently observed in large and comp licated molecular systems. The present paper discusses a possible mech anism to describe the present induction phenomenon. We first show a nu merical fact that an ensemble of trajectories turns into a stage very quickly that can be regarded as a diffusion process getting out of a p otential basin, if projected onto a one-dimensional configuration spac e. Thus, a natural idea arises that the induction time should be a con sequence for the group of trajectories to be transported to the reacti on regions, or transition regions, with a limited speed. In contrast, the standard statistical theories assume that the population in a tran sition region that is lost to the product side is to be supplied insta ntaneously from the reactant region. We present a simple diffusion mod el to examine the above idea. It has been found that the frequencies o f isomerization can be reproduced in a good quantitative level by the estimate in terms of the first passage time based on the calculated di ffusion coefficients and related quantities. The remarkable uniformity of the average passage-times (lifetimes) that was previously found by us is also described well in this simple model. (C) 1998 American Ins titute of Physics.