We provide experimental evidence on the origin of the even-odd rotational state population alternation in the 16O2(a1Δg) 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 1Δg) 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 1Δg) 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.

Understanding the ‘odd’ behavior in ozone photodissociation

ACS Spring Meeting 2021

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