2003ApJ...592L..95C

Precise rest frequencies for the J=1-0 and J=2-1 rotational transitions of C¹⁸O have been measured in the laboratory using the Lamb-dip technique with an accuracy better than 1 kHz (2 σ uncertainty). In contrast to the main CO isotopomer, interstellar C¹⁸O lines are significantly narrower and very often have Gaussian line shapes. Taking into account their relatively high intensities in interstellar clouds, the new laboratory measurements allow us to consider C¹⁸O as the best frequency standard in radio astronomical spectroscopy.