title: Kinetics of the reactions of CH2Br and CH2I radicals with molecular oxygen at atmospheric temperatures.
abstract: 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 x 10(17) 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)x 10(-12)(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+ He)=(1.2 +/- 0.2)x 10(-30)(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 F(cent) value of 0.4. Estimated overall uncertainties in the measured reaction rate coefficients are about +/-25%.
authors.affiliation: Laboratory of Physical Chemistry, University of Helsinki, P.O. Box 55 (A.I. Virtasen aukio 1), FIN-00014, Finland.
authors.forename: Arkke J
authors.initials: AJ
authors.lastname: Eskola
authors.forename: Dorota
authors.initials: D
authors.lastname: Wojcik-Pastuszka
authors.forename: Emil
authors.initials: E
authors.lastname: Ratajczak
authors.forename: Raimo S
authors.initials: RS
authors.lastname: Timonen
mesh_terms: D000388:Air / Q000032:analysis; D001966:Bromine; D006846:Hydrocarbons, Halogenated / Q000737:chemistry*; D007455:Iodine; D007700:Kinetics; D010100:Oxygen; D010782:Photolysis; D013816:Thermodynamics
chemical_list: D006846:Hydrocarbons, Halogenated; D007455:Iodine; D010100:Oxygen; D001966:Bromine