Precise rest frequencies for the first four rotational transitions of 13 CO 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 σ ≤ 1 m s -1 in a Doppler velocity scale. We found that the hyperfine (hf) splitting constant due to 13 C 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 13 CO observations in dark clouds without nonthermal broadening.
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Cazzoli et al. (2004) studied this question.
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