Merged-Beams Reaction Studies of O + H3+

We have measured the reaction of O + H3+ forming OH+ and H2O+. This is believed to be one of the key gasphase astrochemical processes initiating the formation of water molecules in dense molecular clouds. For this work, we have used a novel merged fast-beams apparatus that overlaps a beam of H3 + onto a beam of ground-term neutral O. Here, we present cross-section data for forming OH+ and H2O+ at relative energies from ≈3.5 meV to ≈15.5 and 0.13 eV, respectively. Measurements were performed for statistically populated O (3PJ) in the ground term reacting with hot H3+ (with an internal temperature of ~2500–3000 K). From these data, we have derived rate coefficients for translational temperatures from ≈27 K to ~10^5 K for the formation of OH+ and ~10^3 K for the formation of H2O+. In order to convert these results to a thermal rate coefficient suitable for astrochemistry, we have added the results together for both exit channels and scaled the summed data on statistically populated oxygen to thermally populated oxygen. For this we have used theory as a guide, thereby accounting for the temperature dependence of the O fine-structure levels. Our results are in good agreement with two independent flowing afterglow measurements at a temperature of ≈300 K, and with a corresponding level of H3+ internal excitation. This good agreement suggests that the internal excitation of the H3+ does not play a significant role in this reaction. The Langevin rate coefficient is in easonable agreement with the experimental results at 10 K but a factor of ~2 larger at 300 K. The two published classical trajectory studies using quantum-mechanical potential energy surfaces lie a factor of ~1.5 above our experimental results over this 10–300 K range.