(Funded by the National Science Foundation)

The origin of anomalously large enrichments of stratospheric ozone in heavy isotopes has been a mystery for 30 years. The isotope 16O is dominant in the atmosphere, so that most oxygen molecules (O2) consist of two 16O atoms. However, stratospheric ozone (O3) is surprisingly observed to be heavily enriched in the isotope 18O relative to the atmospheric oxygen from which it’s formed. Careful experimental studies have shown that the recombination reaction that forms ozone,

O2 + O →O3,

is responsible for the effect. The recombination rates for various isotopic combinations differ by more then 50%, which is a remarkably large isotope effect taking into account small mass difference. We propose a clear explanation for the effect in terms of the energy transfer mechanism, where the metastable O3* states of ozone are formed first and then stabilized by collisions with bath gas molecules M:

O2 + O ↔ O3* ↔ O + O2,

O3* + M → O3 + M.

Sophisticated treatment is employed, which considers different metastable O3* states as different species, with their energies and lifetimes obtained from accurate quantum scattering calculations. Population of the metastable O3* states is found to build up and then decay back to O2 + O through several possible channels. When different isotopes of oxygen are involved the channels become open at different energies due to the differences in quantum zero-point energies (ΔZPE) of different O2 molecules.

Figure 1: Schematic for recombination processes forming 16O18O18O isotopologue. The PES (dotted line), quantum ZPE for each entrance channels and the ΔZPE are shown (not to scale). The experimental formation rates (relative to the formation rate of 16O16O16O ) for each channel are given at the top. Note one extra arrow in the ΔZPE energy range.

We found that the spectrum of metastable O3* states is anomalously dense within the ΔZPE energy range, and these states are accessible only from the lower entrance channel. Also, the metastable states in the ΔZPE part of spectrum are stabilized very efficiently by collisions with M because they are energetically close to the bound O3 states (see Fig. 1). Such processes significantly enhance the formation rates of ozone isotopologues through the lower energy channels. These findings have finally identified the molecular level origin of anomalous isotope effects observed in many experiments.

Current group activities involve further theoretical study of this and other quantum phenomena using the mixed quantum/classical method. Our purpose is to provide the most rigorous and complete description of the ozone forming reaction, including all associated isotope effects. Several other atmospheric  reactions like

should exhibit a similar isotope effect. These reactions will be studied as well. 

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