The experimental measurements were taken from
(1) G. Klapper, F. Lewen, R. Gendriesch, S. P. Belov, and G. Winnewisser 2000, J. Mol. Spectrosc., 201, 124.
The higher frequency transitions (> 2 THz) from
(2) L. R. Zink, P. Natale, F. S. Pavone, M. Prevedelli, K. M. Evenson, and M. Inguscio, 1990, J. Mol. Spectrosc., 143, 304,
were also used in the fit.
With respect to the April 2000 entry, data were included which come from
(3) G. Cazzoli, C. Puzzarini, and A. V. Lpinov, 2004, Astrophys. J., 611, 615.
The dipole moment and its centrifugal distortion corrections were taken from
(4) D. Goorvitch 1994, Astrophys. J. Suppl. 95, 535.
The hyperfine splitting caused by the ¹³C nucleus may be relevant in quiescent sources for low values of J. Therefore, a separate hyperfine calculation is provided for J" up to 2. The hyperfine splitting has been taken into account in the calculation of the partition function.