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

Abstract

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.