REACTION STUDIES of NEUTRAL ATOMIC C with H3+USING A MERGED-BEAMS APPARATUS

Abstract

We have investigated the chemistry of C + H3+ forming CH+, CH2+, and CH3+. 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 CH2+ at relative energies from ≈9 meV to ≈20 and 3 eV, respectively. Measurements were performed for statistically populated C (3PJ) in the ground term reacting with hot H3+ (at an internal temperature of ∼2550 K). Using these data, we have derived rate coefficients for translational temperatures from ≈72 K to 2.3 105 ∼ × and 3.4 104 × K, respectively. For the formation of CH3+, we are only able to place an upper limit on the rate coefficient. Our results for CH+and CH2+ are in good agreement with the mass-scaled results from a previous ion trap study of C + D3+, at a translational temperature of ∼1000 K. That work also used statistically populated C (3PJ) but internally cold D3+ (∼77 K). The good agreement between the two experiments implies that the internal excitation of the H3 + 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 CH2+ 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 CH2+ formation dominates over that of CH+ at temperatures ≲50 K.