Experimental determination of the rate constants of the reactions of HO 2 + DO 2 and DO 2 + DO 2

The rate constants of the reactions of DO 2  + HO 2 (R1) and DO 2  + DO 2 (R2) have been determined by the simultaneous, selective, and quantitative measurement of HO 2 and DO 2 by continuous wave cavity ring‐down spectroscopy (cw‐CRDS) in the near infrared, coupled to a radical generation by laser photolysis. HO 2 was generated by photolyzing Cl 2 in the presence of CH 3 OH and O 2 . Low concentrations of DO 2 were generated simultaneously by adding low concentrations of D 2 O to the reaction mixture, leading through isotopic exchange on tubing and reactor walls to formation of low concentrations of CH 3 OD and thus formation of DO 2 . Excess DO 2 was generated by photolyzing Cl 2 in the presence of CD 3 OD and O 2 , small concentrations of HO 2 were always generated simultaneously by isotopic exchange between CD 3 OD and residual H 2 O. The rate constant k 1 at 295 K was found to be pressure independent in the range 25–200 Torr helium, but increased with increasing D 2 O concentration k 1  = (1.67 ± 0.03) × 10 −12  × (1 + (8.2 ± 1.6) × 10 −18  cm 3  × [D 2 O] cm −3 ) cm 3 s −1 . The rate constant for the DO 2 self‐reaction k 2 has been measured under excess DO 2 concentration, and the DO 2 concentration has been determined by fitting the HO 2 decays, now governed by their reaction with DO 2 , to the rate constant k 1 . A rate constant with insignificant pressure dependence was found: k 2  = (4.1 ± 0.6) × 10 −13 (1 + (2 ± 2) × 10 −20  cm 3  × [He] cm −3 ) cm 3 s −1 as well as an increase of k 2 with increasing D 2 O concentration was observed: k 2  = (4.14 ± 0.02) × 10 −13  × (1 + (6.5 ± 1.3) × 10 −18  cm 3  × [D 2 O] cm −3 ) cm 3 s −1 . The result for k 2 is in excellent agreement with literature values, whereas this is the first determination of k 1 .