Precise laboratory frequencies for the J=1-0 and J=2-1 rotational transitions of C18O

Precise rest frequencies for the J = 1-0 and J = 2-1 rotational transitions of (CO)-O-18 have been measured in the laboratory using the Lamb- dip technique with an accuracy better than 1 kHz (2 sigma uncertainty). In contrast to the main CO isotopomer, interstellar (CO)-O-18 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 (CO)-O-18 as the best frequency standard in radio astronomical spectroscopy.