Reactions forming C(0,+)n=2,10, Cn=2,4H(0,+) and C3H(0,+) in the gas phase: semi empirical branching ratios

The aim of this paper is to provide a new set of branching ratios for interstellar and planetary chemical networks based on a semi empirical 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 branching ratios for their parametriza- tion. 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 Cn=2,10, Cn=2,4 H, C3 H2, C+n=2,10, Cn=2,4 H+, or C3 H+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 branching ratios in time dependant chemistry for dark clouds and for photodissociation region chemistry with conditions similar to those found in the Horsehead Nebula are discussed.