Kinetics of chlorine oxide radicals using modulated photolysis. Part 2.—ClO and ClOO radical kinetics in the photolysis of Cl2+ O2+ N2 mixtures

The low intensity photolysis of Cl2+ O2+ N2 mixtures has been reinvestigated using molecular modulation spectrometry to observe the radical species ClOO and ClO in the ultraviolet region. The results substantially confirm the work of Johnston and coworkers on this system.The observed kinetic behaviour of ClOO together with kinetic and thermodynamic data from elsewhere lead to a more definitive value for the equilibrium constant K*2(in cm3 molecule units): Cl + O2+ [graphic omitted] ClOO + M; K*2=k2/k–2= 3.7 × 10–28Texp(3217/T). The value of K*2 allowed rate constants for the following reactions of ClOO at 298 K to be evaluated: Cl + ClOO = Cl2+ O2k3= 9.8 × 10–11 cm3 molecule–1 s–1, Cl + ClOO = 2ClO k4= 4.7 × 10–12 cm3 molecule–1 s–1. The uncertainty in these values is ≈±50%.The overall decay rate constant for the disproportionation of ClO radicals exhibited a strong pressure dependence in the range 25–760 Torr and at high pressures gross deviations from the expected second order kinetic behaviour was observed. The pressure dependence of ClO decay was interpreted in terms of a Cl-atom catalysed mechanism involving the Cl2O2 molecule: ClO + ClO + M ⇌ Cl2O2+ M K*6=k6/k–6, Cl + Cl2O2= ClOO + Cl2k7. K*6 was estimated as 1.7 × 10–29T exp (8163/T) cm3 molecule–1 and k7≈ 1 × 10–11 cm3 molecule–1 s–1.