Vibrational Relaxation of Oxygen by Methane, Acetylene, and Ethylene

Abstract

The vibrational relaxation of oxygen containing from ¼% to 2% of CH₄, C₂H₂, and C₂H₄ has been studied by shock‐tube interferometry. From about 450° to 1300°K the vibrational relaxation zone is not affected by chemical reaction, and the observed oxygen relaxation in mixtures containing only small amounts of hydrocarbon may be accounted for by using the standard formula for a binary mixture and assuming the relaxation time in seconds of oxygen dilute in 1 atm of the hydrocarbon to be given by log₁₀τO₂–CH₄ = 18T^—⅓−8.7, log₁₀τO₂–C₂H₂ = 7.5T^—⅓−7.3, and log₁₀τO₂–C₂H₄ = −6.92. For higher temperatures, the exothermic reaction may result in a nonlaminar flow behind an aplanar shock, and the relaxation zone then cannot be resolved interferometrically. For still higher temperatures the relaxation zone is again laminar, but the vibrational excitation of O₂ is accelerated by concurrent chemical reactions in the induction zone. From the observation at lower temperatures that τO₂–CH₄<τCH₄ and τO₂–C₂H₄<τC₂H₄ one is led to conclude that at least below 800°K collisions with O₂ are more effective in the excitation of CH₄ and C₂H₄ than are self‐collisions, and that the excitation of O₂ by collision with a hydrocarbon molecule is through a vibrational exchange reaction with the more easily excited hydrocarbon molecule.