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Reaction Studies of Neutral Atomic C with H3+ using a Merged-beams Apparatus

O'Connor, A. P.; Urbain, X.; Stutzel, J.; Miller, Kenneth; de Ruette, N.; Garrido, M.; Savin, Daniel Wolf

We have investigated the chemistry of C + H_3^+ forming CH+, CH_2^+, and CH_3^+. These reactions are believed to be some of the key gas-phase astrochemical processes initiating the formation of organic molecules in molecular clouds. For this work, we have constructed a novel merged fast-beams apparatus which overlaps a beam of molecular ions onto a beam of ground-term neutral atoms. Here, we describe the apparatus in detail and present cross section data for forming CH+ and CH_2^+ at relative energies from ≈9 meV to ≈20 and 3 eV, respectively. Measurements were performed for statistically populated C (3P_J) in the ground term reacting with hot H_3^+ (at an internal temperature of ~2550 K). Using these data, we have derived rate coefficients for translational temperatures from ≈72 K to ~2.3 X 10^5 and 3.4 X 10^4 K, respectively. For the formation of CH_3^+, we are only able to place an upper limit on the rate coefficient. Our results for CH+ and CH_2^+ are in good agreement with the mass-scaled results from a previous ion trap study of C + D_3^+, at a translational temperature of ~1000 K. That work also used statistically populated C (3P_J) but internally cold D_3^+ (~77 K). The good agreement between the two experiments implies that the internal excitation of the H_3^+ is not significant so long as the reaction proceeds adiabatically. At 300 K, the C fine-structure levels are predicted to be essentially statistically populated, enabling us to compare our translational temperature results to thermal equilibrium calculations. At this temperature, our rate coefficient for forming CH+ lies a factor of ≈2.9 below the Langevin rate coefficient currently given in astrochemical databases, and a factor of ~1.8-3.3 below the published classical trajectory studies using quantum mechanical potential energy surfaces. Our results for CH_2^+ formation at 300 K are a factor of ≈26.7 above these semi-classical results. Astrochemical databases do not currently include this channel. We also present a method for converting our translational temperature results to thermal rate coefficients for temperatures below ~300 K. The results indicated that CH_2^+ formation dominates over that of CH+ at temperatures ~<50 K.

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Title
The Astrophysical Journal Supplement Series
DOI
https://doi.org/10.1088/0067-0049/219/1/6

More About This Work

Academic Units
Astrophysics Laboratory
Published Here
September 5, 2017