NON-THRESHOLD, THRESHOLD, AND NONADIABATIC BEHAVIOR OF THE KEY INTERSTELLAR C + C2H2 REACTION

The C + C2H2 reaction is a key process in interstellar cloud chemistry. In a crossed-beam scattering experiment approaching the low-collision energies that characterize these environments, we determined relative differential cross sections by detecting the H-atom product. High-level ab initio calculations of the reaction energies of two competing pathways, leading to cyclic and linear C3H, were also performed. Both channels are clearly distinguishable: the integral cross section of the c-C3H + H channel monotonically decreases with increasing relative translational energy whilst the l-C3H + H channel exhibits a translational energy threshold. Moreover, a comparison of the H-atom yields from the C + C2H2 and C + C2H4 reactions shows that the C-3 + H-2 nonadiabatic channel dominates. These results are consistent with the calculated enthalpies and corroborate earlier low-temperature kinetic experiments. Branching ratios of the three reaction pathways are given in the T = 15-300 K temperature domain for inclusion in astrochemical databases.