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

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, N2, O2, CO2 and SF6 at pressures ranging from 10 to 600 Torr. The final products Cl2O4, Cl2O6, Cl2 and O2 are formed. The amount of Cl2O4 and Cl2O6 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 ClClO2 have been detected. The quantum yields for Cl2O4 and Cl2O6 formation satisfy the condition Φ(Cl2O4) + Φ(Cl2O6) ≅ 1 throughout. In the absence of an inert gas Φ(Cl2O4) decreases from a value close to 1 to about 0.5 as OClO pressure is decreased, indicating the formation of Cl2O4 by the reaction ClO3 + ClO → Cl2O4. For about 5 Torr OClO mean pressure and in the presence of an inert gas, Φ(Cl2O4) increases as total pressure is increased reaching a value close to 1 for 300 Torr of N2 or 50 Torr of SF6. At the higher inert gas pressures Φ(Cl2O4) 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 Cl2O5* generated in the process ClO3* + OClO → Cl2O5* → ClO4 + ClO. The generally accepted reaction for the formation of Cl2O6, namely 2ClO3 → Cl2O6, can be ruled out and a complicated mechanism is needed to account for the experimental observations.