Experimental determination of the rate constants of the reactions of HO2 + DO2 and DO2 + DO2

The rate constants of the reactions of DO2 + HO2 (R1) and DO2 + DO2 (R2) have been determined by the simultaneous, selective, and quantitative measurement of HO2 and DO2 by continuous wave cavity ring-down spectroscopy (cw-CRDS) in the near infrared, coupled to a radical generation by laser photolysis. HO2 was generated by photolyzing Cl-2 in the presence of CH3OH and O-2. Low concentrations of DO2 were generated simultaneously by adding low concentrations of D2O to the reaction mixture, leading through isotopic exchange on tubing and reactor walls to formation of low concentrations of CH3OD and thus formation of DO2. Excess DO2 was generated by photolyzing Cl-2 in the presence of CD3OD and O-2, small concentrations of HO2 were always generated simultaneously by isotopic exchange between CD3OD and residual H2O. 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 D2O concentration k(1) = (1.67 +/- 0.03) x 10(-12) x (1 + (8.2 +/- 1.6) x 10(-18) cm(3) x [D2O] cm(-3)) cm(3) s(-1). The rate constant for the DO2 self-reaction k(2) has been measured under excess DO2 concentration, and the DO2 concentration has been determined by fitting the HO2 decays, now governed by their reaction with DO2, to the rate constant k(1). A rate constant with insignificant pressure dependence was found: k(2) = (4.1 +/- 0.6) x 10(-13) (1 + (2 +/- 2) x 10(-20) cm(3) x [He] cm(-3)) cm(3) s(-1) as well as an increase of k(2) with increasing D2O concentration was observed: k(2) = (4.14 +/- 0.02) x 10(-13) x (1 + (6.5 +/- 1.3) x 10(-18) cm(3) x [D2O] 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).