Study of the recombination reaction methyl + methyl .fwdarw. ethane. 1. Experiment

Abstract

The recombination of methyl radicals has been studied as a function of temperature and bath gas density to obtain more detailed knowledge of the kinetics of this reaction in the unimolecular falloff region. The reaction was studied by two techniques: (1) use of a tubular reactor coupled to a photoionization mass spectrometer; (2) use of absorption spectroscopy with a stainless steel flow cell. In both cases, methyl radicals were generated homogeneously by the pulsed photolysis of acetone at 193 nm. CH/sub 3/ decay profiles were monitored in real-time experiments. Absolute CH/sub 3/ concentrations were obtained from direct measurements of acetone photodecomposition in the photoionization experiments and from previously determined cross sections in the absorption experiments. Rate constants for the CH/sub 3/ + CH/sub 3/ reaction are reported, with argon as the bath gas, at nine temperatures in the range 296-906 K and at gas densities from 1.8 x 10/sup 16/ to 1.4 x 10/sup 19/ molecules cm/sup -3/. A significantly narrower range of conditions was used in additional experiments using helium as the bath gas. The results obtained are compared with theoretical values of the CH/sub 3/ + CH/sub 3/ recombination rate constant obtained in part 2 (following paper inmore » this issue). The combined results of the experimental and theoretical study of this reaction provide a more quantitative description of the role of collisions in this important recombination process. The uncertainty associated with extrapolating unimolecular rate constants to higher temperatures by using a common functional representation of the pressure and temperature dependence of such rate constants is illustrated with a comparison of theory and extrapolatioon up to 2000 K for the CH/sub 3/ + CH/sub 3/ high-pressure-limit rate constant.« less