Reactions of halogen oxides studied by flash photolysis II. The flash photolysis of chlorine monoxide and of the ClO free radical

A study of the flash photolysis of chlorine monoxide and of its photosensitized decomposition by chlorine and bromine has yielded rate constants for the reactions Cl + Cl2O → Cl2 + ClO, k1 = 4.1 x 108 l mol-1 s-1, Br + Cl2O → BrCl + ClO, k9 = 6.1 x 108 l mol-1 s-1, ClO + Cl2O → ClO2 + Cl2, k3 = 2.6 x 105 l mol-1 s-1, ClO + Cl2O → Cl2 + O2 + Cl, k4 = 6.5 x 105 l mol-1 s-1, 2ClO → Cl2 + O2, k2 = 2.8 x 107 l mol-1 s-1. The quantum yield for the decomposition of chlorine monoxide was measured in each of the three systems and is quantitatively accounted for by the reactions given. The CIO free radical has been flash photolysed and the production of vibrationally excited oxygen in the reaction O + CIO → Cl + O*2 (v" ≼ 14), k11 = 7.5 x 109 l mol-1 s-1 demonstrated. The same reaction is responsible for the production of O*2 in the flash photo­lysis of Cl2O with radiation below ~ 300 nm. The relaxation of O*2 by chlorine atoms is exceptionally efficient, with a rate constant for v" = 12 in excess of 2 x 109 l mol-1 s-1. The corresponding rate constant for relaxation by Cl2O is < 108 l mol-1 s-1.