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 inve stigated in the temperature range 285-331 K using time-resolved diode array spectroscopy to monitor the decay of O-3 and the formation of OC lO. The branching ratio was derived at each temperature from the measu red 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 an d the following branching ratios into the three bimolecular disproport ionation 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 expe nse 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, whil e k(3a)/k(3c) did not exhibit Arrhenius type temperature dependence. T he validity of the methods used was verified by the determination of t he 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 recommen ded kinetic data.