Branching Ratio for O + H3 + Forming OH+ + H2 and H2O+ + H

The gas-phase reaction of O+H3+ has two exothermic product channels: OH+ + H2 and H2O+ + 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% versus 30% for the formation of OH+ + H2 versus H2O+ + 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% versus 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(3PJ) reactant is in its J = 2 ground state and the other one where it is in thermal equilibrium.