Rate constants and molecular recombination pathways of oxygen from quasi-classical trajectory simulations of theO3system

Abstract

• Investigation of recombination mechanisms using quasi-classical trajectory method. • Direct collisions are more effective for recombination. • Indirect recombinations proceed through orbiting pairs. • Recombination products result in strong non-Boltzmann internal energy distributions. • Recombination products exhibit large vibrational and low rotational quantum numbers. A lack of understanding of the dynamics of molecular recombination has led to the development of a large number of recombination models and methods to quantify recombination rate constants, each with their own set of assumptions and ranges of applicability. This work aims to unravel the complex dynamics behind the recombination of atomic oxygen to molecular oxygen using the quasi-classical trajectory method for the O 3 system. Recombination pathways considered include a single-step “direct” reaction, and a two-step mechanism that involves the formation of a long-lived intermediate which is stabilized through collision with a third body. The latter is further classified as Lindemann or Chaperon recombination pathways depending on the identity of the intermediate and the final product. The classical versions of all three mechanisms are explored in detail for the O 3 system. A time-lag parameter is introduced to quantify the time interval between the intermediate formation and third-body stabilization steps, and the direct collisions are treated as a limiting case when the time lag is zero. It is found that the recombination probabilities reduce as the time-lag is increased. Direct recombination events are dominated by head-on collisions, while the two step recombination events are characterized by a combination of glancing and head-on collisions. The resulting recombination products follow a strongly non-Boltzmann distribution. Recombination rate coefficients are computed through a novel rate coefficient formula that takes into consideration the Poisson distribution in time of third body stabilizing collisions with the orbiting pair characterized by finite lifetimes (on the order of picoseconds) that lead to recombination. Good agreement is obtained with recombination rates computed from detailed balance based on known dissociation rate coefficients. This approach thus enables the direct calculation of recombination rate coefficients at low temperatures, without the need for dissociation rate coefficients and detailed balance.