Quasi Classical Trajectory Analysis of Oxygen Recombination Using a Consistent Binary Lifetime Framework

The dissociation and recombination of diatomic molecules in shock layers around high speed vehicles have first order effects on the chemical and thermal energy fluxes to those vehicles’ heat shields. The Direct Molecular Simulation (DMS) technique has been used to study the dissociation process in detail, and results have shown the existence of binary collisions that last for very long times.[1–3] This work studies the recombination process using the Quasi Classical Trajectory (QCT) method to determine the importance of these long-lived collisions. Recombination reactions require the collision of three bodies, which can be broadly grouped into two types: the direct collision of three particles, and the multi-step collision of an interacting pair of particles with a third. Recent work presented triple collision rate constants constructed from binary collision lifetimes without a direct appeal to detailed balance, but found that the lifetime definition used was not adequate to describe both types of triple collisions.[4] This work uses a lifetime based three body collision rate constant that allows any binary collision to result in either type of triple collision. Distributions of binary lifetimes are studied to show that most binary collisions are short, lasting less than 20000 fs, and that while binary collisions lasting longer are very rare, they become more prevalent for O + O₂ collisions at low temperatures. The recombination probabilities for three different O₂ formation pathways are compared, and the resulting total recombination rate constants are within an order of magnitude of predictions from detailed balance. Our results indicate that very long-lived binary collisions are exceedingly rare, and do not contribute significantly to the recombination process at temperatures ranging from 1000 K-5000 K in oxygen.