CH₂OO Criegee intermediate UV absorption cross-sections and kinetics of CH₂OO + CH₂OO and CH₂OO + I as a function of pressure

The UV absorption cross-sections of the Criegee intermediate CH2OO, and kinetics of the CH2OO self-reaction and the reaction of CH2OO with I are reported as a function of pressure at 298 K. Measurements were made using pulsed laser flash photolysis of CH2I2/O2/N2 gas mixtures coupled with time-resolved broadband UV absorption spectroscopy at pressures between 6 and 300 Torr. Results give a peak absorption cross-section of (1.37 ± 0.29) × 10−17 cm2 at ∼340 nm and a rate coefficient for the CH2OO self-reaction of (8.0 ± 1.1) × 10−11 cm3 s−1, with no significant pressure dependence of the absorption cross-sections or the self-reaction kinetics over the range investigated. The rate coefficient for the reaction between CH2OO and I demonstrates pressure dependence over the range investigated, with a Lindemann fit giving k0 = (4.4 ± 1.0) × 10−29 cm6 s−1 and k∞ = (6.7 ± 0.6) × 10−11 cm3 s−1. The origins of IO in the system have been investigated, the implications of which are discussed.