EDP Sciences
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Volume 365, Number 2, January II 2001
Page(s) 241 - 247
Section Physical and chemical processes
DOI http://dx.doi.org/10.1051/0004-6361:20000026

A&A 365, 241-247 (2001)
DOI: 10.1051/0004-6361:20000026

Rate coefficients for the reactions of C( $\mathsf{^3}$P $\mathsf{{\it _J}}$) atoms with C $\mathsf{_2}$H $\mathsf{_2}$, C $\mathsf{_2}$H $\mathsf{_4}$, CH $\mathsf{_3}$C $ \mathsf{\equiv} $CH and H $\mathsf{_2}$C $ \mathsf{=} $C $ \mathsf{=} $CH $\mathsf{_2}$ at temperatures down to 15 K

D. Chastaing , S. D. Le Picard , I. R. Sims and I. W. M. Smith .

School of Chemistry, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK

(Received 29 August 2000 / Accepted 11 October 2000)

Rate coefficients for the reactions of ground state carbon atoms, C(3PJ), with the four unsaturated hydrocarbons C2H2, C2H4, ${\rm CH_3C\equiv CH}$ and ${\rm H_2C=C=CH_2}$ have been measured at temperatures down to 15 K. The experiments have been performed in a continuous flow CRESU (Cinétique de Réaction en Écoulement Supersonique Uniforme) apparatus using pulsed laser photolysis of C3O2 to generate C(3PJ) atoms and laser-induced fluorescence in the vacuum ultraviolet to observe the kinetic decays of the atoms and hence determine the rate coefficients. All four reactions are found to occur at rates close to the collision-determined limit and the rates show at most a very mild dependence on temperature. It is argued that normally these reactions proceed along a reaction path with no barrier to form a strongly bound energised adduct, which subsequently eliminates an H atom to form the reaction products. The rates of the reactions of C(3PJ) atoms with larger unsaturated hydrocarbons are discussed. Finally, the relevance of the results for the chemistry of dense interstellar clouds is considered.

Key words: molecular data -molecular processes -methods: laboratory -ISM: clouds

Offprint request: I. W. M. Smith, i.w.m.smith@bham.ac.uk

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