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

The rate coefficient for the recombination of methyl radicals, CH 3 + CH 3 → C 2 H 6, 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 × 10 18 and 1.1 × 10 19 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 k dis ( T ) = 2.22 × 10 39 T -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)} cm 3 s -1, k 0 ( T ) = 1.70 × 10 -5 T -7.248 exp{−2172/ T (K)} cm 6 s -1, and F cent ( T ) = exp{− T (K)/506}.