Unimolecular decomposition kinetics of the stabilised Criegee intermediates CHOO and CDOO

Decomposition kinetics of stabilised CH 2 OO and CD 2 OO Criegee intermediates have been investigated as a function of temperature (450-650 K) and pressure (2-350 Torr) using flash photolysis coupled with time-resolved cavity-enhanced broadband UV absorption spectroscopy. Decomposition of CD 2 OO was observed to be faster than CH 2 OO under equivalent conditions. Production of OH radicals following CH 2 OO decomposition was also monitored using flash photolysis with laser-induced fluorescence (LIF), with results indicating direct production of OH in the v = 0 and v = 1 states in low yields. Master equation calculations performed using the Master Equation Solver for Multi-Energy well Reactions (MESMER) enabled fitting of the barriers for the decomposition of CH 2 OO and CD 2 OO to the experimental data. Parameterisations of the decomposition rate coefficients, calculated by MESMER, are provided for use in atmospheric models and implications of the results are discussed. For CH 2 OO, the MESMER fits require an increase in the calculated barrier height from 78.2 kJ mol −1 to 81.8 kJ mol −1 using a temperature-dependent exponential down model for collisional energy transfer with 〈Δ E 〉 down = 32.6( T /298 K) 1.7 cm −1 in He. The low- and high-pressure limit rate coefficients are k 1,0 = 3.2 × 10 −4 ( T /298) −5.81 exp(−12 770/ T ) cm 3 s −1 and k 1,∞ = 1.4 × 10 13 ( T /298) 0.06 exp(−10 010/ T ) s −1 , with median uncertainty of ∼12% over the range of experimental conditions used here. Extrapolation to atmospheric conditions yields k 1 (298 K, 760 Torr) = 1.1 +1.5 −1.1 × 10 −3 s −1 . For CD 2 OO, MESMER calculations result in 〈Δ E 〉 down = 39.6( T /298 K) 1.3 cm −1 in He and a small decrease in the calculated barrier to decomposition from 81.0 kJ mol −1 to 80.1 kJ mol −1 . The fitted rate coefficients for CD 2 OO are k 2,0 = 5.2 × 10 −5 ( T /298) −5.28 exp(−11 610/ T ) cm 3 s −1 and k 2,∞ = 1.2 × 10 13 ( T /298) 0.06 exp(−9800/ T ) s −1 , with overall error of ∼6% over the present range of temperature and pressure. The extrapolated k 2 (298 K, 760 Torr) = 5.5 +9.2 −5.5 × 10 −3 s −1 . The master equation calculations for CH 2 OO indicate decomposition yields of 63.7% for H 2 + CO 2 , 36.0% for H 2 O + CO and 0.3% for OH + HCO with no significant dependence on temperature between 400 and 1200 K or pressure between 1 and 3000 Torr. Decomposition kinetics of stabilised CH 2 OO and CD 2 OO Criegee intermediates have been investigated as a function of temperature (4