A robust dipole moment of carbon monoxide (CO) is a permanent puzzle for both spectroscopic and ab initio studies

The asymptotically correct semi-empirical dipole moment function (DMF) for the ground X1Σ+ state of the CO molecule was redetermined in an analytical form for the entire range of interatomic distances r∈[0,+∞] by a global least-squared fitting both experimental intensities and ab initio data. The region r∈[0.8,1.9] Å that is governed exclusively by the empirical data was broadened by adding experimental relative intensity ratios measured in the emission from high vibrational levels v. Absolute absorption intensities recently obtained for very high 7-0 overtone and accompanied sub-percent measurements for 3-0 band were also included in the fitting procedure. The present DMF reproduces a vast majority of the experimental intensities up to v≤38 within accuracy of measurements, except ‘abnormally’ sensitive 5-0 band, and systematically overestimates the most previous ab initio and semi-empirical functions at r≥1.5 Å . Possible reasons of the observed discrepancies are throughout discussed. The resulting DMF can be used, along with mass-invariant interatomic potential, to upgrade rovibrational line lists for any isotopologues of the CO molecule up to the dissociation threshold. The corresponding FORTRAN subroutine is provided.