Reaction Studies of Neutral Atomic $\mathrm{C}$ with ${\mathrm{H}}_3^{+}$ using a Merged-Beams Apparatus

Abstract

We have investigated the chemistry of $\mathrm{C}+{\mathrm{H}}_3^{+}$ 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 present cross section data for forming CH${}^{+}$ and CH${}_2^{+}$ at collision energies from $\approx 9$ meV to $\approx 20$ and 3 eV, respectively. Using these data we have derived thermal rate coefficients for reaction temperatures from $\approx 72$ K to $\approx 2.3\times 10^5$ and $3.4\times 10^4$ K, respectively. For the formation of CH${}_3^{+}$ we are able only to put 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 $\mathrm{C}+{\mathrm{D}}_3^{+}$ at a reaction temperature of $\sim 1000$ K. At molecular cloud temperatures our thermal rate coefficient for forming CH${}^{+}$ lies a factor of $\sim 2-4$ below the Langevin rate coefficient currently given in astrochemical databases and below the published semi-classical calculations. Our results for CH${}_2^{+}$ formation are a factor of $\sim 26$ above the semi-classical results. Astrochemical databases do not currently include this channel.

Accepted for publication in the Astrophysical Journal Supplement

References in corpus (2)

• Chemistry in Protoplanetary Disks: A Sensitivity Analysis
• A sensitivity study of the neutral-neutral reactions C + C3 and C + C5 in cold dense interstellar clouds

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