Neutral–neutral reactions at the temperatures of interstellar clouds: Rate coefficients for reactions of atomic carbon, C(3P), 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 coefficients for the reactions of ground-state carbon atoms, C(3P), with C2H2, C2H4, C3H6 and O2 at temperatures from 295 down to 15 K. C(3P) atoms were generated by photolysis of C3O2 at 193 nm using an ArF excimer laser and reaction rates were determined by observing the chemiluminescence from NO(B2Π) which is generated in the reaction between C(3P) atoms and NO2. The rate coefficients for all four reactions increase as the temperature is lowered, and those for reactions with the unsaturated hydrocarbons exceed 2×10-10 cm3 molecule-1 s-1 at all temperatures below 300 K. The results confirm that C(3P) atoms, like C2H radicals and CN radicals, react rapidly with unsaturated hydrocarbons at very low temperatures. It therefore seems likely that these reactions—and similar ones involving C(3P) atoms and other hydrocarbons—play an important role in forming species with long carbon chains in dense interstellar clouds.