Isomer-specific kinetics of the C⁺ + H₂O reaction at the temperature of interstellar clouds

The reaction C⁺ + H₂O → HCO⁺/HOC⁺ + H is one of the most important astrophysical sources of HOC⁺ ions, considered a marker for interstellar molecular clouds exposed to intense ultraviolet or x-ray radiation. Despite much study, there is no consensus on rate constants for formation of the formyl ion isomers in this reaction. This is largely due to difficulties in laboratory study of ion-molecule reactions under relevant conditions. Here, we use a novel experimental platform combining a cryogenic buffer-gas beam with an integrated, laser-cooled ion trap and high-resolution time-of-flight mass spectrometer to probe this reaction at the temperature of cold interstellar clouds. We report a reaction rate constant of k = 7.7(6) × 10^-9 cm³ s^-1 and a branching ratio of formation η = HOC⁺/HCO⁺ = 2.1(4). Theoretical calculations suggest that this branching ratio is due to the predominant formation of HOC⁺ followed by isomerization of products with internal energy over the isomerization barrier.