The kinetics of the reactions of CH2Br and CH2I radicals with O2 have been studied in direct measurements using a tubular flow reactor coupled to a photoionization mass spectrometer. The radicals have been homogeneously generated by pulsed laser photolysis of appropriate precursors at 193 or 248 nm. Decays of radical concentrations have been monitored in time-resolved measurements to obtain the reaction rate coefficients under pseudo-first-order conditions with the amount of O2 being in large excess over radical concentrations. No buffer gas density dependence was observed for the CH2I + O2 reaction in the range 0.2–15 × 1017 cm−3 of He at 298 K. In this same density range the CH2Br + O2 reaction was obtained to be in the third-body and fall-off area. Measured bimolecular rate coefficient of the CH2I + O2 reaction is found to depend on temperature as k(CH2I + O2)
(1.39 ± 0.01)
(T/300 K)−1.55 ± 0.06 cm3 s−1
(220–450 K). Obtained primary products of this reaction are I atom and IO radical and the yield of I-atom is significant. The rate coefficient and temperature dependence of the CH2Br + O2 reaction in the third-body region is k(CH2Br + O2
(1.2 ± 0.2)
(T/300 K)−4.8 ± 0.3 cm6 s−1
(241–363 K), which was obtained by fitting the complete data set simultaneously to a Troe expression with the Fcent value of 0.4. Estimated overall uncertainties in the measured reaction rate coefficients are about ±25%.
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