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, 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 x 10(18) and 1.1 x 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 x 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(infinity)(T) = 1.53 x 10(-7) T--1.203 exp{-295/T(K)} cm(3) s(-1), k(0)(T) = 1.70 x 10(-5) T--7.248 exp{-2172/T(K)} cm(6) s(-1), and F-cent(T) = exp{-T(K)/506}.