Irregular semi-empirical dipole-moment function for carbon monoxide and line lists for all its isotopologues verified for extremely high overtone transitions

Abstract

In our recent publication [JQSRT, 2021, 272, 107803] (Medvedev21), we discussed the problem of which restrictions must be imposed onto the diatomic functions in order that they would be capable of predicting, at least in part, the intensities of yet unobserved lines. The most general requirement to fulfill is that they must obey all known theoretical restrictions. Recently, the first steps in this direction were made by constructing, for carbon monoxide, a semi-empirical fully analytical mass-corrected potential-energy function (PEF) [Meshkov et al. JQSRT, 2018, 217, 262] and a mass-independent dipole-moment function (DMF), Medvedev21 and [Meshkov et al., JQSRT, 2022, 280, 108090], with correct behavior in the limits of small and large inter-atomic separations. Both the PEF and DMF were fitted not only to the experimental line positions and transition moments but also to their ab initio values. A few additional requirements to the DMF were formulated, including the straight NIDL of [Medvedev, J. Chem. Phys., 2012, 137, 174307] and stability of predictions with respect to variations of the model DMF parameters caused by variations of the database to which the model was fitted. In this paper, the irregular DMF with branch points proposed in Medvedev21 is modified by increasing the rate of reaching the long-range limit and also by decreasing the number of adjustable parameters. In order to ensure the reliability of the predictions, a few regular model DMFs with no singularities in any finite part of the complex plane and a rational DMF are considered for the purpose of comparison. The idea behind such an approach is that similar intensity predictions of the DMFs with cardinally different analytical properties will testify the validity of the results obtained and will provide for a rough estimate of the prediction errors. The line lists for all CO isotopologues are generated in the range of quantum numbers v′=0-41, Δv≤15,J≤150 (J≤200 for 0-0 and 1-0 bands). The validity of the calculated intensities up to Δv=15 is demonstrated. Estimates of the expected errors of the calculated line intensities in the 7-0 band are given.