Precise laboratory frequencies for the J←J-1 (J=1, 2, 3, 4) rotational transitions of 13CO

Precise rest frequencies for the first four rotational transitions of (CO)-C-13 have been measured in the laboratory using the Lamb-dip technique with an accuracy better than 1 kHz. Taking into account available frequencies for the higher J transitions, the new measurements allowed us to predict rotational transitions up to 1 THz with 2 sigma less than or equal to 1 m s(-1) in a Doppler velocity scale. We found that the hyperfine (hf) splitting constant due to C-13 magnetic spin-rotation interaction, estimated from Lamb-dip measurements, is in excellent agreement with the results from molecular-beam experiments. In contrast to recent studies, no rotational dependence on the J quantum number has been determined with significantly high accuracy. Since hf splitting is comparable to the thermal line broadening at T-k = 10 K, its analysis can be important for 13CO observations in dark clouds without nonthermal broadening.