13C18O

13C18O is the isotopologue of carbon monoxide (CO) in which the carbon atom is the isotope 13C and the oxygen atom is 18O. Like CO, it is a heteronuclear diatomic molecule with a small permanent electric dipole moment, which allows pure rotational transitions and produces a spectrum in the infrared and the millimeter/submillimeter radio region.

The substitution of heavier isotopes increases the molecular mass and moment of inertia, causing small shifts in rotational transition frequencies compared with the main isotopologue. Rest frequencies are determined by laboratory spectroscopy and are essential for identifying lines in astronomical spectra. Because 13C18O formation depends on two rare isotopes, its natural abundance is very low—typically of order 10^−5 relative to the dominant 12C16O species—making its lines faint and requiring sensitive observations.

In astronomy and planetary science, 13C18O serves as a tool to probe molecular gas where the principal CO isotopologue is optically thick. Its emission enables more accurate estimates of column densities and helps determine isotopic abundance ratios such as 12C/13C and 16O/18O in various environments, including star-forming regions, molecular clouds, and extragalactic systems. Observational studies rely on high-sensitivity radio/millimeter facilities to detect the weak lines and to calibrate chemical and isotopic evolution in the interstellar medium.

Laboratory measurements and astronomical detections of 13C18O contribute to models of CO chemistry and isotopic fractionation, supporting broader insights into the composition and evolution of astrophysical environments.