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

TL;DR

This paper introduces a semi-empirical statistical model to generate new branching ratios for interstellar chemical reactions, improving accuracy over existing database values and impacting astrochemical models of dark clouds and nebulae.

Contribution

The paper develops a semi-empirical microcanonical model for reaction branching ratios, validated against experimental data, and applies it to astrochemical reaction networks.

Findings

Model predictions align with experimental measurements.

Significant deviations from existing database values.

Impacts on astrochemical models of dark clouds and nebulae.

Abstract

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.