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Merged-beams Reaction Studies of O + H_3^+

We have measured the reaction of O + H3+ forming OH+ and H2O+. This is one of the key gas-phase astrochemical processes initiating the formation of water molecules in dense molecular clouds. For this work, we have used a novel merged fast-beams apparatus which 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 \approx 3.5 meV to \approx 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 \approx 2500-3000 K). From these data, we have derived rate coefficients for translational temperatures from \approx 25 K to \approx 10^5 and 10^3 K, respectively. Using state-of-the-art theoretical methods as a guide, we have converted these results to a thermal rate coefficient for forming either OH+ or H2O+, 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 \approx 300 K, and with a corresponding level of H3+ internal excitation. This good agreement strongly suggests that the internal excitation of the H3+ does not play a significant role in this reaction. The Langevin rate coefficient is in reasonable agreement with the experimental results at 10 K but a factor of \approx 2 larger at 300 K. The two published classical trajectory studies using quantum mechanical potential energy surfaces lie a factor of \approx 1.5 above our experimental results over this 10-300 K range.