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.

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