We investigated the reaction C2(X 1Σ+ g /a 3Π u )?+ C2H4 at collision energy 5.0?kcal?mol–1 in a crossed molecular-beam apparatus using selective photoionization. Time-of-flight and photoionization spectra of products C4H3 and C4H2 were 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–1, respectively. Product C4H3 is identified as H2CCCCH because its ionization threshold 8.0?± 0.2?eV and maximal translational-energy release 42?kcal?mol–1 coincide with that of product channel H2CCCCH?+ H. H2CCCC (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–1 is within the energetic limits of both isomeric product channels. Nonetheless, channel H2CCCC?+ H2 is 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 C2H4 are abundant.
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