Formation of molecular oxygen in ultracoldO+OHcollisions

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 ${\mathrm{O}}_{2}$ molecules are reported. A $J$-shifting approximation is used to compute the rate coefficients. At temperatures $T=10--100\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$ 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, $Tl1\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$, 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 $T=0.3\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, at higher temperatures, the quantum calculations predict a larger rate coefficient than the classical model, in agreement with experimental data for the $\mathrm{O}+\mathrm{O}\mathrm{H}$ reaction. The zero-temperature limiting value of the rate coefficient is predicted to be about $6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{3}\phantom{\rule{0.2em}{0ex}}{\text{molecule}}^{\ensuremath{-}1}\phantom{\rule{0.2em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$, 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 $\mathrm{F}+{\mathrm{H}}_{2}$ reaction.