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

Abstract

A CRESU (Cine′tique de Re′action en Ecoulement Supersonique Uniforme) apparatus has been used to measure rate constants for the reactions of the ethynyl radical (C₂H) with O₂, 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(A²Δ) which is generated in a minor channel of the reaction between C₂H radicals and O₂. The rate constants for all four reactions increase as the temperature is lowered, and those for reactions with the unsaturated hydrocarbons exceed 10^-10 cm³ 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.