Reactions Forming C_{n=2,10}^{(0,+)}, C n = 2, 4H(0, +), and C3H_2^{(0,+)} in the Gas Phase: Semiempirical Branching Ratios
Résumé
The aim of this paper is to provide a new set of branching ratios (BRs) for interstellar and planetary chemical networks based on a semiempirical model. We applied, instead of zero-order theory (i.e., only the most exoergic decaying channel is considered), a statistical microcanonical model based on the construction of breakdown curves and using experimental high velocity collision BRs for their parameterization. We applied the model to ion-molecule, neutral-neutral, and ion-pair reactions implemented in the few popular databases for astrochemistry, such as KIDA, OSU, and UMIST. We studied the reactions of carbon and hydrocarbon species with electrons, He+, H+, CH+, CH, C, and C+ leading to intermediate complexes of the type C n = 2, 10, C n = 2, 4H, C3H2, C_{n=2,10}^+, C n = 2, 4H+, or C3H_2^+. Comparison of predictions with measurements supports the validity of the model. Huge deviations with respect to database values are often obtained. Effects of the new BRs in time-dependent chemistry for dark clouds and for photodissociation region chemistry with conditions similar to those found in the Horsehead Nebula are discussed.