Precise Laboratory Frequencies for theJ= 1-0 andJ= 2-1 Rotational Transitions of C18O

Precise rest frequencies for the J = 1-0 and J = 2-1 rotational transitions of C 18 O 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 C 18 O 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 C 18 O as the best frequency standard in radio astronomical spectroscopy.