The transition frequencies were taken from laboratory spectroscopy as well as astronomical observations by
(1) W. G. D. P. Silva, J. Cernicharo, S. Schlemmer, N. Marcelino, J.-C. Loison, M. Agúndez, D. Gupta, V. Wakelam, S. Thorwirth, C. Cabezas, B. Tercero, J. L. Doménech, R. Fuentetaja, W.-J. Kim, P. de Vicente, and O. Asvany,
2023, Astron. Astrophys. 676, Art. No. L1.
Omission of the highest laboratory frequency resulted in a fit of reasonable quality. Transition frequencies with K_a ≥ 2 should be viewed with increasing caution. Transition frequencies with K_a ≤ 1 should be sufficiently accurate up to at least 300 GHz.
The molecular ion exists in ortho and para forms with an intensity ratio of 3 : 1. The para levels are described by K_a being even. The dipole moment is from a quantum-chemical calculation by
(2) X. Huang and T. J. Lee, 2011, Astrophys. J. 736, Art. No. 33.