Kinetics of chlorine oxide radical reactions using modulated photolysis. Part 1.—Disproportionation of ClO in the Cl2 photosensitised decomposition of ozone

Abstract

ClO radicals were produced by the photolysis at 298 K of mixtures of Cl₂ and O₃ diluted in 1 atm (760 Torr) O₂, using a square-wave modulated light source (λ= 360±50 nm). The formation and decay of ClO and OClO were monitored by time-resolved ultraviolet absorption spectrophotometry. A differential absorption arrangement was used to subtract background absorptions due to O₃ and Cl₂, allowing direct measurements of the transient species concentrations. Quantum yields for the photosensitised decomposition of ozone were also measured.

ClO decayed by three parallel bimolecular reaction channels and the rate constants for these reactions were obtained by computer simulations of the experimental concentration time data for ClO, OClO and O₃: ClO + ClO = Cl₂+ O₂k_3a= 4.5 ± 1 × 10^–15 cm³ molecule^–1 s^–1, ClO + ClO = Cl + ClOO k_3b= 3.1 ± 0.8 × 10^–15 cm³ molecule^–1 s^–1, ClO + ClO = Cl + OClO k_3c= 1.5 ± 0.4 × 10^–15 cm³ molecule^–1 s^–1. The behaviour of ClO showed significant departure from second-order kinetics under modulated photolysis and this can be interpreted in terms of the formation of a dimer of ClO which has sufficient stability at 298 K to be kinetically significant ClO + ClO + M ⇌ ClOOCl + M. (4) An estimate of the equilibrium constant K^*₄≈ 10^–14 cm³ molecule^–1 at 298 K was obtained.