The kinetics and the mechanism of ozone photolysis at 253.7 nm

Abstract The photolysis of ozone by steady illumination at 253.7 nm and 25 °C was studied using two different techniques. In one the ozone decomposition from an initial pressure of 50 Torr was measured manometrically at constant volume and in the other the decomposition from an initial pressure of 1 – 2 Torr was measured photometrically by monitoring changes in the optical absorption. In both cases the light intensity was measured during each irradiation period. The experiments were performed in pure ozone and in ozone containing oxygen, helium, sulphur hexafluoride and nitrogen. The quantum yield Φ of the decomposition of pure ozone was 5.68 ± 0.15 molecules quantum −1 and was pressure independent. The quantum yields Φ He and Φ SF 6 of the decomposition of ozone in HeO 3 ( p He / p O 3 ≈ 600) and SF 6 O 3 ( p SF 6 / p O 3 ≈ 300) were 5.62 ± 0.41 molecules quantum −1 and 5.44 ± 0.32 molecules quantum −1 respectively. In N 2 O 3 mixtures with high values of p N 2 / p O 3 the quantum yield decreases to 4 molecules quantum −1 . Oxygen has a strong inhibiting effect. All the experimental data can be quantitatively explained using the following mechanism: The quantum yield φ for O( 1 D) production is 0.92 ± 0.04 atoms quantum −1 . The unspecified O 2 * molecule is formed with an efficiency of 0.47 ± 0.15. Under our experimental conditions O 2 * molecules are highly resistant to collisional deactivation and most are removed via reaction (4a). The efficiency of deactivation of O( 1 D) atoms by N 2 is 0.19 ± 0.01. A value of 0.58 was found for k 7a k 8a / k 7 k 8 .