A robust dipole moment of carbon monoxide (CO) is a permanent puzzle for both spectroscopic and <i>ab initio</i> 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 <i>ab initio</i> 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 <i>v</i>. 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 <i>ab initio</i> 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.