2012ApJ...759...75L

We investigated the reaction C₂(X ¹Σ⁺_g/a ³Π_u ) + C₂H₄ at collision energy 5.0 kcal/mol in a crossed molecular-beam apparatus using selective photoionization. Time-of-flight and photoionization spectra of products C₄H₃ and C₄H₂were measured. From the best simulation of product time-of-flight spectra, a low-energy-biased translational-energy distribution and an isotropic angular distribution are derived for product channels C₄H₃+ H and C₄H₂+ H₂ that have average translational-energy releases of 11 and 20 kcal/mol, respectively. Product C₄H₃ is identified as H₂ CCCCH because its ionization threshold 8.0±0.2 eV and maximal translational-energy release 42 kcal/mol coincide with that of product channel H₂ CCCCH + H. H₂ CCCC (butatrienylidene) and HCCCCH (diacetylene) might have contributions to product C₄H₂; both isomers have ionization energies near the measured ionization threshold 10.0±0.2 eV and the maximal translational-energy release 62 kcal/mol is within the energetic limits of both isomeric product channels. Nonetheless, channel H₂ CCCC + H₂is suggested to be more dominant than channel HCCCCH + H₂ because the maximal translational-energy release is in good agreement with the available energy of the former channel and the former channel is 3.8 times the branching ratio of the later channel predicted by Rice-Ramsperger-Kassel-Marcus calculations. C₄H₂ is identified for the first time in the barrierless reaction C₂+ C₂H₄ which has never been considered in any astronomical chemical networks. This work sheds new light on the formation of butatrienylidene/diacetylene in cold interstellar media where C₂ and C₂H₄are abundant.