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 at 248, 266, and 282 nm. 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 alternation from the jet-cooled precursor is much stronger than that observed previously by Valentini et al., and in contrast to their observations does not depend strongly on O₂ (a ¹Δ_g) vibrational state or photolysis wavelength. The odd/even alternation diminishes substantially when the ozone beam temperature is increased from 60 to 200 K, confirming its dependence on parent internal energy. We have also measured temperature-dependent vector correlations from O₃ photodissociation at 266 nm using ion imaging. The magnitude of the even/odd alternation in product rotational states from the cold ozone sample, its temperature dependence, and other experimental and theoretical evidence reported since 1987 suggest that the alternation originates from a Λ-doublet propensity, and not from a mass independent curve crossing effect as previously proposed. For non-rotating ozone, its dissociation on the excited B¹A’ state dictates that only A’ Λ-doublets are populated, due to symmetry conservation. This selection rule is relaxed for rotating parent molecules, but a preference still persists for A’ Λ-doublets. The A′′/A′ ratio increases with increasing ozone rotational quantum number, and thus with increasing temperature, explaining the previously observed temperature dependence of the even-odd population alternation. 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.

Simon North

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