Recombination of Methyl Radicals. 1. New Data between 1175 and 1750 K in the Falloff Region

Abstract

The rate coefficient for the recombination of methyl radicals, CH3 + CH3 → C2H6, was measured in incident shock-wave experiments on azomethane (0.12−1.3%) in argon. Methyl radicals were detected at 46 294 cm-1 (215.94 nm in air) by the tunable-laser flash-absorption technique. Postshock pressures between 114 and 230 kPa (1.13−2.27 atm) produced buffer gas concentrations between 5.4 × 1018 and 1.1 × 1019 cm-3. The cross section for optical absorption, the rate of dissociation of azomethane, and the rate coefficient of the recombinational reaction were determined simultaneously by least-squares analysis of methyl absorption profiles. The rate coefficient for the dissociation of azomethane from 850 to 1430 K is kdis(T) = 2.22 × 1039T-7.99 exp{−25920/T(K)} s-1. The temperature-dependent rate of recombination at 1.2 atm compares favorably with previous measurements. In terms of the phenomenological three-parameter equation of Kooji and the asymmetric Lorentzian broadening function of Gilbert et al., the recombination data from 296 to 1800 K may be described by k∞(T) = 1.53 × 10-7 T-1.203 exp{−295/T(K)} cm3s-1, k0(T) = 1.70 × 10-5T-7.248 exp{−2172/T(K)} cm6 s-1, and Fcent(T) = exp{−T(K)/506}.