Rate coefficients for the reactions of C(P-3(J)) 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 Ecoulem ent Supersonique Uniforme) apparatus using pulsed laser photolysis of C3O2 to generate C(P-3(J)) atoms and laser-induced fluorescence in the vacuum ul traviolet to observe the kinetic decays of the atoms and hence deter mine t he 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 de pendence on temperature. It is argued that normally these reactions proceed along a reaction path with no barrier to form a strongly bound energised a dduct, which subsequently eliminates an H atom to form the reaction product s. The rates of the reactions of C(P-3(J)) atoms with larger unsaturated hy drocarbons are discussed. Finally, the relevance of the results for the che mistry of dense interstellar clouds is considered.