Rate and equilibrium constants for the reaction of HO2 with formaldehyde
and acetaldehyde
have been directly measured. The concentration of HO2 radicals was followed in a time‐resolved method by coupling cw‐CRDS (cavity ringdown spectroscopy) to laser photolysis. The reaction of HO2 with CH2O (R1) was measured at 50 Torr helium over the temperature range 292–306 K, whereas the reaction of CH3CHO with HO2 (R2) was measured in 50 Torr He but at only 294 K. The observed HO2 decay profiles were modeled to take into account secondary chemistry, especially that of the reaction products, hydroxyl‐peroxy radical adducts. The rate constants for forward and back reactions of (R1) at 297 K were found to be k1 = (3.3 ± 0.6) × 10−14 cm3 molecule−1 s−1 and k−1 = (55 ± 5) s−1, respectively, both roughly a factor of two slower than earlier measurements (possibly due to falloff effects), while the equilibrium constant was found to be K1 = (6.0 ± 1.8) × 10−16 molecule−1 cm3 at 297 K, in good agreement with earlier, more indirect determinations. The equilibrium constant of the reaction with CH3CHO was found to be K2 = (1.7 ± 0.5) × 10−17 molecule−1 cm3 at 294 K, with the forward rate constant k2 = (1.5 ± 0.75) × 10−14 cm3 molecule−1 s−1 and the rate constant for the back reaction k−2 = (900 ± 450) s−1.