SEARCHING FOR INTERSTELLAR MOLECULE BUTATRIENYLIDENE IN REACTION C2+ C2H4

Abstract

We investigated the reaction C2(X1Σ+g/a3Πu) + 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.