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

TL;DR

This study analyzes the temperature-dependent branching ratios of the gas-phase reaction between oxygen and H3+ ions, providing new rate coefficients and showing the ratio varies significantly from previous assumptions, especially at low temperatures.

Contribution

It offers the first detailed temperature-resolved branching ratios and rate coefficients for the reaction, improving astrochemical models by accounting for temperature effects.

Findings

Branching ratio varies from 70% to 58% for OH+ formation as temperature decreases.

Provides recommended rate coefficients for different initial fine-structure populations.

Results are consistent with existing data at 295 K but reveal temperature dependence at lower temperatures.

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$(^3P_J)$ reactant is in its $J=2$ ground state and the other one where it is in thermal equilibrium.