Insights into the Formation of Hydroxyl Radicals with Nonthermal Vibrational Excitation in the Meinel Airglow

Abstract

To understand night time airglow in the Meinel bands and heat conversion from the highly excited OH radicals in the upper atmosphere via the important atmospheric reaction H + O3 → OH + O2, we report here a quasi-classical trajectory study of the reaction dynamics on a recently developed full-dimensional potential energy surface (PES). Our results indicate that the reaction energy of this highly exoergic reaction is almost exclusively channeled into the vibration of the OH product, underscoring an extreme departure from the statistical limit. The calculated OH vibrational distribution is highly inverted and peaks near the highest accessible vibrational state, in excellent agreement with experimental observations, validating the accuracy of the PES. More importantly, the dynamical origin of the nonthermal excitation of the OH vibrational mode is identified by its large projection onto the reaction coordinate at a small potential barrier in the entrance channel, which controls the energy flow into various degrees of freedom in the products.

Reaction Pathway Control via Reactant Vibrational Excitation and Impact on Product Vibrational Distributions: The O + HO2 → OH + O2 Atmospheric Reaction

“…25,26 This new PES has been demonstrated to reproduce various spectroscopic, kinetic, and dynamics properties of the system. 22,23,27,28 In several comparisons, the results on the new PES were found to differ significantly from those obtained from the DMBE PESs, 17,19 presumably due to the neglect of multireference effects in the earlier ab initio calculations and the relatively small number of ab initio points used in the DMBE fitting. As a result, it is not clear if the previous dynamics results for the title reaction obtained from the DMBE PESs are reliable.…”

“…For situations at thermal equilibrium, the nightglow is most likely due to the H + O 3 → OH(v) + O 2 reaction, which occurs on the same PES. 27 In summary, we investigated the detailed dynamics of the O + HO 2 → OH + O 2 reaction using a QCT method on a recently developed PIP-NN PES to obtain nascent rovibrational distributions of the two products and to understand their dynamical origins. To explore vibrational control of two reaction pathways to the same products, we show that the excitation of the v 1 vibrational mode of HO 2 opens the HA channel, which has a higher barrier, and significantly enhances the reaction rate.…”

Quantitative molecular simulations

“…190,207 Also, for the H + O 3 → OH + O 2 reaction, QCT simulations based on a PIP + NN representation of energies calculated at the MRCI level of theory reported final state vibrational distributions for the OH product peaking at v ′ = 8, in near-quantitative agreement with available experiments. 208…”

“…Phys., 2022, 24, 12767-12786 | 12773 representation of energies calculated at the MRCI level of theory reported final state vibrational distributions for the OH product peaking at v 0 = 8, in near-quantitative agreement with available experiments. 208 Finally, thermal rates for reactions have also been determined for bimolecular processes. As an example, the kinetics of the OH + HO 2 reaction to form O 2 + H 2 O was determined from QCT simulations using a full dimensional PES based on PIP + NN.…”

Quantitative Molecular Simulations

“…163,180 Also, for the H+O 3 → OH+O 2 reaction, QCT simulations based on a PIP+NN representation of energies calculated at the MRCI level of theory reported final state vibrational distributions for the OH product peaking at v = 8, in near-quantitative agreement with available experiments. 181 Finally, thermal rates for reactions have also been determined for bimolecular processes. As an example, the kinetics of the OH+HO 2 reaction to form O 2 +H 2 O was determined from QCT simulations using a full dimensional PES based on PIP + NN.…”