The relative quantum yields of O₂(a¹Δg) from the photolysis of ozone at 227K

The relative quantum yields of O 2 2(a¹Δ g ) from the photolysis of ozone have been measured at 227(±2)K over the wavelength range 300nm ≥ λ ≥ 322nm, and are compared to those measured previously at 298K [ Ball et al., 1993]. The yields at the two temperatures show the same initial wavelength dependence in the fall‐off region, in contrast to literature measurements of the O(¹D) yields which show a shift in the fall‐off curve to shorter wavelengths at low temperature. For λ > 309nm, the yield at 227K falls more rapidly than that at 298K, confirming that internal energy mediated dissociation of ozone provides the major source of O 2 (a¹Δ g ) at wavelengths just beyond the 310nm threshold for the spin allowed production of the two singlet products. However, the yields of O 2 (a¹Δ g ) at 227K and 298K were found to be approximately equal for photolysis wavelengths λ ≥ 320nm, and substantially larger than O(¹D) yields predicted from modelling currently available for the photolysis of vibrationally excited ozone. These results suggest the existence of a spin forbidden dissociation channel for O 2 (a¹Δ g ) with a quantum yield of Φ ≈ 0.1 at wavelengths near to 320nm.