The photochemical decomposition of gaseous OClO at 436 nm: Dependence of products distribution on total pressure

Abstract

A systematic experimental study of the photochemical decomposition of 5–80 Torr gaseous OClO at 293 and 303 K has been performed. The photolysis was also investigated in the presence of the added inert gases He, Ar, N 2, O 2, CO 2 and SF 6 at pressures ranging from 10 to 600 Torr. The final products Cl 2O 4, Cl 2O 6, Cl 2 and O 2 are formed. The amount of Cl 2O 4 and Cl 2O 6 was determined by IR and UV spectrophotometry, respectively. Both products reach a limit of about 30% of the total yield. Only very small amounts of ClClO 2 have been detected. The quantum yields for Cl 2O 4 and Cl 2O 6 formation satisfy the condition Φ(Cl 2O 4) + Φ(Cl 2O 6) ≅ 1 throughout. In the absence of an inert gas Φ(Cl 2O 4) decreases from a value close to 1 to about 0.5 as OClO pressure is decreased, indicating the formation of Cl 2O 4 by the reaction ClO 3 + ClO → Cl 2O 4. For about 5 Torr OClO mean pressure and in the presence of an inert gas, Φ(Cl 2O 4) increases as total pressure is increased reaching a value close to 1 for 300 Torr of N 2 or 50 Torr of SF 6. At the higher inert gas pressures Φ(Cl 2O 4) slightly decreases when OClO is increased. This result is attributed to the interplay established between reactive and collisional deactivation process in the excited radical intermediate Cl 2O 5 * generated in the process ClO 3 * + OClO → Cl 2O 5 * → ClO 4 + ClO. The generally accepted reaction for the formation of Cl 2O 6, namely 2ClO 3 → Cl 2O 6, can be ruled out and a complicated mechanism is needed to account for the experimental observations.