Branching ratio for $\text{O}+{\text{H}}_3^{+}$ forming ${\text{OH}}^{+}+{\text{H}}_2$ and ${\text{H}}_2{\text{O}}^{+}+\text{H}$

Abstract

The gas-phase reaction of $\mathrm{O}+{\mathrm{H}}_3^{+}$ has two exothermic product channels, ${\mathrm{OH}}^{+}+{\mathrm{H}}_2$ and ${\mathrm{H}}_2{\mathrm{O}}^{+}+\mathrm{H}$. In the present study, we analyze experimental data from a merged-beams measurement to derive thermal rate coefficients resolved by product channel for the temperature range from 10 to 1000 K. Published astrochemical models either ignore the second product channel or apply a temperature-independent branching ratio of 70% vs. 30% for the formation of ${\mathrm{OH}}^{+}+{\mathrm{H}}_2$ vs. ${\mathrm{H}}_2{\mathrm{O}}^{+}+\mathrm{H}$, respectively, which originates from a single experimental data point measured at 295 K. Our results are consistent with this data point, but show a branching ratio that varies with temperature reaching 58% vs. 42% at 10 K. We provide recommended rate coefficients for the two product channels for two cases, one where the initial fine-structure population of the O${(}^3{P}_J)$ reactant is in its $J=2$ ground state and the other one where it is in thermal equilibrium.

7 pages, 2 figures, 2 tables

References in corpus (7)

• The 2014 KIDA network for interstellar chemistry
• Herschel Survey of Galactic OH+, H2O+, and H3O+: Probing the Molecular Hydrogen Fraction and Cosmic-Ray Ionization Rate
• Water in star-forming regions (WISH): Physics and chemistry from clouds to disks as probed by Herschel spectroscopy
• Chemistry of TMC-1 with multiply deuterated species and spin chemistry of H2, H2+, H3+ and their isotopologues
• Reaction Studies of Neutral Atomic $\mathrm{C}$ with ${\mathrm{H}}_3^{+}$ using a Merged-Beams Apparatus
• The ortho-to-para ratio of water in interstellar clouds
• Experimental and theoretical studies of D + H${}_3^{+}$ $\to$ H${}_2$D${}^{+}$ + H