Formation of molecular oxygen in ultracold O + OH reaction

TL;DR

This paper investigates the formation of molecular oxygen in ultracold O + OH reactions using quantum calculations, revealing temperature-dependent reaction pathways and validating classical models at very low temperatures.

Contribution

It introduces a quantum formalism for ultracold O + OH reactions and compares it with classical models, providing new insights into reaction dynamics at near-zero temperatures.

Findings

Reactive and elastic rates are comparable at 10-100 mK.

Oxygen formation dominates below 1 mK.

Quantum results align with experiments at 0.3 K.

Abstract

We discuss the formation of molecular oxygen in ultracold collisions between hydroxyl radicals and atomic oxygen. A time-independent quantum formalism based on hyperspherical coordinates is employed for the calculations. Elastic, inelastic and reactive cross sections as well as the vibrational and rotational populations of the product O2 molecules are reported. A J-shifting approximation is used to compute the rate coefficients. At temperatures T = 10 - 100 mK for which the OH molecules have been cooled and trapped experimentally, the elastic and reactive rate coefficients are of comparable magnitude, while at colder temperatures, T < 1 mK, the formation of molecular oxygen becomes the dominant pathway. The validity of a classical capture model to describe cold collisions of OH and O is also discussed. While very good agreement is found between classical and quantum results at T=0.3 K, at higher temperatures, the quantum calculations predict a larger rate coefficient than the classical model, in agreement with experimental data for the O + OH reaction. The zero-temperature limiting value of the rate coefficient is predicted to be about 6.10^{-12} cm^3 molecule^{-1} s^{-1}, a value comparable to that of barrierless alkali-metal atom - dimer systems and about a factor of five larger than that of the tunneling dominated F + H2 reaction.