Sub-Doppler measurements and rotational spectrum of (CO)-C-13-O-18

The rotational spectrum of (CO)-C-13-O-18 has been measured up to 1 THz. Th e lowest rotational transitions of (CO)-C-13-O-18 (J = 2 <-- 1 to J = 6 <-- 5) have been measured by saturation-dip spectroscopy with an experimental accuracy of 2 kHz. These five low J rotational transitions cover the freque ncy range between 209 and 628 GHz. The narrow linewidths of about 20 kHz of the saturation digs allowed to resolve the two main hyperfine components. The splitting is caused by coupling of the C-13 nuclear spin with the rotat ion of the molecule. The appropriate coupling constant C-1((CO)-C-13-O-18) is 33.90(81) kHz. In addition we have measured in the Doppler limited mode, the line position s of the rotational transitions J = 7 <-- 6, J = 8 <-- 7, and J = 9 <-- 8 w ith accuracies of 5 kHz. We provide a set of improved constants together wi th frequency predictions up to 4.1 THz (J = 40 <-- 39).