Understanding the ‘Odd’ Behavior in Ozone Photodissociation*

We provide experimental evidence on the origin of the even-odd rotational state population alternation in the ¹⁶O₂(a¹Δ_g) fragments resulting from the UV photodissociation of ¹⁶O₃, a phenomenon first observed over 30 years ago. We have measured rotational distributions for the O₂ (a ¹Δ_g) fragment from photodissociation of jet-cooled O₃ in the UV. The rotational distributions show a population alternation that favors the even states, as previously reported for a 300 K sample by Valentini et al. (J. Chem. Phys. 86, 6745 (1987)). The odd/even alternation diminishes substantially when the ozone beam temperature is increased from 60 to 200 K. We have also measured temperature-dependent vector correlations from O₃ photodissociation at 266 nm using ion imaging. The measurements suggest that the alternation originates from a Λ-doublet propensity, and not from a mass independent curve crossing effect as previously proposed. We find that a simply classical model can reproduce all the observed experimental trends. In light of these results, it is concluded that the previously proposed parity-selective curve-crossing mechanism cannot be a source of heavy isotopic enrichment in the atmosphere.