Femtosecond real-time probing of reactions. 7. A quantum- and classical-mechanical study of the cyanogen iodide dissociation experiment

Abstract

A comparison is presented between experiment and quantum and classical simulations of the time-dependent dynamics of the dissociation reaction of ICN via the A continuum. To illustrate the approach, second-order perturbation theory and a classical model (Ber. Bunsen-Ges. Phys. Chem. 1988, 92, 373) are employed to calculate the real-time behavior of dissociating [I⋅⋅⋅CN]^(••) species during the course of the reaction. In this way, the abilities of the two methods to describe accurately the reaction dynamics as revealed experimentally are evaluated: it is found that both quantum dynamical and classical mechanical treatments are capable of reproducing the essential temporal features probed by experiment and that, in this instance, classical equations of motion offer an adequate description of the process of nuclear separation. The difference between the relevant excited-state potential functions (over the long-range region) employed in the calculations is recovered from the simulated data by means of an inversion procedure (J. Chem. Phys. 1989, 90, 829) that relates absorption of the probing laser pulse to interfragment distance along the reaction coordinate. In addition, the variation in calculated transient behavior resulting from changes in the parameters describing the functional forms of the two potentials governing the observed reaction dynamics is examined in terms of their effect on characteristic reaction dissociation times, the lifetimes of transition-state configurations, and inversion to the difference between potential energy curves. Finally, comparison is made with analogous experimental and theoretical investigations of the fragmentation of the heavier Bi_2 molecule.