NEUTRAL-NEUTRAL REACTIONS AT THE TEMPERATURES SF INTERSTELLAR CLOUDS - RATE COEFFICIENTS FOR REACTIONS OF C2H RADICALS WITH O-2, C2H2, C2H4AND C3H6 DOWN TO 15 K

A CRESU (Cinetique de Reaction en Ecoulement Supersonique Uniforme) ap paratus has been used to measure rate constants for the reactions of t he 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 C2 H2 at 193 nm using an ArF excimer laser and reaction rates are determi ned by observing the chemiluminescence from CH(A (2)Delta) which is ge nerated in a minor channel of the reaction between C2H radicals and O- 2. The rate constants for all four reactions increase as the temperatu re is lowered, and those for reactions with the unsaturated hydrocarbo ns exceed 10(-10) cm(3) molecule(-1) s(-1) at all temperatures below 1 00 K. The results confirm that C2H radicals, like CN radicals, react r apidly 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.