An experimental and theoretical investigation of the competition between chemical reaction and relaxation for the reactions of 1CH2 with acetylene and ethene: implications for the chemistry of the giant planets

Abstract

The temperature dependence of the branching ratios for H atom production from the reactions of the first excited state of methylene (a¹A₁¹CH₂) with acetylene and ethene have been measured at ∼1 Torr total pressure and temperatures of 195, 250 and 298 K by monitoring the production of H atoms using laser induced fluorescence, comparing the signal to that observed from a calibration reaction. For the reaction with acetylene the yield of H increases from 0.28 (195 K) to 0.53 (250 K) to 0.88 at 298 K. The H atom yield from the reaction of ¹CH₂ with ethene shows similar behaviour, the yields being 0.35 (195 K), 0.51 (250 K) and 0.71 (298 K). The co-products, propargyl (C₃H₃) and allyl (C₃H₅) are formed from the dissociation of chemically activated C₃H₄ and C₃H₆ intermediates respectively, and are important species in the formation of higher hydrocarbons, including benzene, in the atmospheres of the outer planets and Titan. H atom production is in competition with electronic relaxation to form ground state methylene (X³B₁, ³CH₂) and collisional stabilization to form C₃H₄ and C₃H₆. Master equation calculations have been carried out to demonstrate that for the reaction of ¹CH₂ with acetylene, collisional stabilization is insignificant under experimental conditions and hence the balance of reaction is due to electronic relaxation. Non-adiabatic transition state theory has been applied to the reaction of ¹CH₂ with acetylene. The calculations show reasonable agreement with experiment, generally being within the combined errors, and reproduce the negative temperature dependence for electronic relaxation. The implications of the temperature dependence of the absolute rate coefficients for ¹CH₂ reactions with inert gases, hydrogen, acetylene and ethene and of the branching ratios between chemical reaction and electronic relaxation are discussed.