A combined fit of v = 0, v₁₁ = 1 μ²Σ, and v₁₁ = 1 ²Δ was performed. The FTMW and millimeter data were published by
(1) C. A. Gottlieb, M. C. McCarthy, and P. Thaddeus, 2010, Astrophys. J. Suppl. Ser. 189, 261.
Perturbations between v = 0 and v₁₁ = 1 μ²Σ appear to be negligible. The parameter b_F was identical within uncertainties for the three vibrational states, and the values were thus locked to the same value. Additional distortion parameters were applied in comparison to (1). Two transition frequencies with large residuals were omitted. The origin was kept fixed to the value from
(2) D. Zhao, M. A. Haddad, H. Linnartz, and W. Ubachs, 2011, J. Chem. Phys. 135, Art. No. 044307.
The calculation was truncated at 300 GHz, transition frequencies should be reliable up to that frequency.
At low frequencies, the ¹H hyperfine splitting may be resolvable in space. Therefore, a separate calculation with hyperfine splitting is available up to 51 GHz. NOTE: the partition function does take into account the hyperfine splitting as well as the two excited vibrational states!
The dipole moment is from an ab initio calculation by
(3) D. E. Woon, 1995, Chem. Phys. Lett. 244, 45.
Please note that the dipole moment may be slightly overestimated because of some mixing between the ²Π_i and the ²Σ state.