Rate coefficients for the reactions of C(3PJ) atoms with C2H2, C2H4, CH3C=CHand H2C=C=CH2 at temperatures down to 15 K

Rate coefficients for the reactions of ground state carbon atoms, C(P-3(J)), With the four unsaturated hydrocarbons C2H2, C2H4, CH3C = CH and H2C = C = CH2 have been measured at temperatures down to 15 K. The experiments have been performed in a continuous Row CRESU (Cinetique de Reaction en Ecoulement Supersonique Uniforme) apparatus using pulsed laser photolysis of C3O2 to generate C(P-3(J)) atoms and laser-induced fluorescence in the vacuum ultraviolet to observe the kinetic decays of the atoms and hence deter mine 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(P-3(J)) atoms with larger unsaturated hydrocarbons are discussed. Finally, the relevance of the results for the chemistry of dense interstellar clouds is considered.