Neutral-neutral reactions at the temperatures of interstellar clouds: Ratecoefficients for reactions of atomic carbon, C(P-3), with O-2, C2H2, C2H4 and C3H6 down to 15 K

A CRESU (Cinetique de Reaction en Ecoulement Supersonique Uniforme) apparat us has been used to measure rate coefficients for the reactions of ground-s tate carbon atoms, C(P-3), with C2H2, C2H4, C3H6 and O-2 at temperatures fr om 295 down to 15 K. C(P-3) atoms were generated by photolysis of C3O2 at 1 93 nm using an ArF excimer laser and reaction rates were determined by obse rving the chemiluminescence from NO(B (II)-I-2) which is generated in the r eaction between C(P-3) atoms and NO2. The rate coefficients for all four re actions increase as the temperature is lowered, and those for reactions wit h the unsaturated hydrocarbons exceed 2 x 10(-10) cm(3) molecule(-1) s(-1) at all temperatures below 300 K. The results confirm that C(P-3) atoms, lik e C2H radicals and CN radicals, react rapidly with unsaturated hydrocarbons at very low temperatures. It therefore seems likely that these reactions-a nd similar ones involving C(P-3) atoms and other hydrocarbons-play an impor tant role in forming species with long carbon chains in dense interstellar clouds.