Neutral-neutral reactions at the temperatures sf interstellar clouds -: Rate coefficients for reactions of C2H radicals with O2, C2H2, C2H4 and C3H6 down to 15 K

A CRESU (Cinetique de Reaction en Ecoulement Supersonique Uniforme) apparatus has been used to measure rate constants for the reactions of the ethynyl radical (C2H) with O-2, C2H2, C2H4 and C3H6 at temperatures from 295 down to 15 K. C2H radicals are generated by photolysis of C2H2 at 193 nm using an ArF excimer laser and reaction rates are determined by observing the chemiluminescence from CH(A (2)Delta) which is generated in a minor channel of the reaction between C2H radicals and O-2. The rate constants for all four reactions increase as the temperature is lowered, and those for reactions with the unsaturated hydrocarbons exceed 10(-10) cm(3) molecule(-1) s(-1) at all temperatures below 100 K. The results confirm that C2H radicals, like CN radicals, react rapidly with unsaturated hydrocarbons at very low temperatures and, in both cases, the radical replaces an H atom in the unsaturated molecule. It therefore seems likely that combinations of these reactions play a major role in forming species with long C chains and in synthesising the cyanopolyynes in dense interstellar clouds.