2025ApJS..276...66M

The Measured Active Rotational-Vibrational Energy Levels (MARVEL) algorithm is used to determine accurate empirical rotational-vibrational energy levels for the ground electronic state of the diatomic ¹²C¹⁶O molecule. 2293 energy levels have been obtained through a careful analysis of lines measured and assigned in high-resolution experimental spectra reported previously in 68 publications. Out of the 19,399 (7955) measured (unique) transitions in the limited wavenumber range of 0-14,470 cm^–1, an analysis of the resulting experimental spectroscopic network (SN) validates 19,219 (7795), and only 11 transitions had to be deleted from the SN assembled (note that transitions within floating components of the SN cannot be validated). The measured transitions span states with vibrational and rotational quantum numbers less than or equal to 41 and 123, respectively, with the highest validated energy level lying at 67,148.1 cm^–1. The validation procedure covers all transitions with a one-photon absorption intensity larger than 10^–30 cm molecule^–1 at 296 K. The validated line centers and the empirical rovibrational energy levels of ¹²C¹⁶O, with appropriate uncertainties and assignments, are provided in the appendix to this paper. Detailed comparisons are made with several existing data sets, such as the Kurucz and the HITRAN databases, NIST-certified wavelengths, and the list of lines protected by the International Astronomical Union, revealing occasional discrepancies.