On the role of rotation-vibration coupling in the spectra of ozone isotopomers

TL;DR

This paper develops a theoretical and computational framework to accurately analyze the rotational-vibrational spectra of ozone isotopomers, assessing the impact of Coriolis coupling on isotopic fractionation.

Contribution

It introduces a new code using hyper-spherical coordinates and optimized basis sets for precise spectra calculations, and evaluates the Coriolis effect's role in ozone isotopic enrichment.

Findings

Coriolis effect in ozone is relatively small.

Deviations from symmetric-top behavior are minor.

Mass and symmetry influence spectral characteristics equally.

Abstract

A theoretical framework and computer code are developed for accurate calculations of coupled rotational-vibrational states in triatomic molecules using hyper-spherical coordinates and taking into account the Coriolis coupling effect. Concise final formulae are derived for construction of the Hamiltonian matrix using an efficient combination of the FBR and DVR methods with locally optimized basis sets and grids. First, the new code is tested by comparing its results with those of the APH3D program of Kendrick. Then, accurate calculations of the ro-vibrational spectra are carried out for doubly substituted symmetric (18O16O18O) and asymmetric (18O18O16O) ozone isotopomers for total angular momentum up to J=5. Together with similar data recently reported for the singly substituted symmetric (16O18O16O) and asymmetric (16O16O18O) ozone isotopomers, these calculations quantify the role of the Coriolis coupling effect in the large mass-independent isotopic enrichment of ozone, observed in both laboratory experiments and the atmosphere of Earth. It is found that the Coriolis effect in ozone is relatively small, as evidenced by deviations of its rotational constants from the symmetric-top-rotor behavior, by the magnitudes of parity splittings ({\Lambda}-doubling), and by the ratios of ro-vibrational partition functions for asymmetric vs. symmetric ozone molecules. It is concluded that all of these characteristics are influenced by the isotopic masses as much as they are influenced by the overall symmetry of the molecule. It is therefore unlikely that the Coriolis coupling effect could be responsible for symmetry-driven mass-independent fractionation of oxygen isotopes in ozone.