TEMPERATURE-DEPENDENCE OF THE PRODUCT BRANCHING RATIOS OF THE CLO SELF-REACTION IN OXYGEN

The Cl-2-sensitized continuous photolysis of O-3-O-2 mixtures was investigated in the temperature range 285-331 K using time-resolved diode array spectroscopy to monitor the decay of O-3 and the formation of OClO. The branching ratio was derived at each temperature from the measured quantum yield of ozone consumption (Phi(-03)) and the initial rate of OClO formation. At 298 K Phi(-03) was found to be 4.04 +/- 0.35 and the following branching ratios into the three bimolecular disproportionation channels of the ClO self-reaction were determined: k(3a)/k(3) = 0.39 +/- 0.06, k(3b)/k(3) = 0.41 +/- 0.06, and k(3c)/k(3) = 0.20 +/- 0.03, where ClO + ClO --> Cl-2 + O-2 (3a), ClO + ClO --> Cl + ClOO (3b), and ClO + ClO --> Cl + OClO (3c). The contribution of the radical channels 3b and 3c was found to increase with temperature at the expense of the molecular channel 3a. The temperature dependence of k(3b)/k(3c) can be described by the following apparent Arrhenius parameters, A(3c)/A(3b) = 0.27 +/- 0.10 and E(3c) - E(3b) = 1.79 +/- 0.78 kJ, while k(3a)/k(3c) did not exhibit Arrhenius type temperature dependence. The validity of the methods used was verified by the determination of the equilibrium constant K-4, where ClO + ClO + M <----> Cl2O2 + M (4), which was found to equal (2.24 +/- 0.35) x 10(-14) cm(3) molecule(-1) at 285 K, in excellent agreement with the value derived from recommended kinetic data.