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

Abstract

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.