The Ω = 4 spin component is the lowest in energy. Individual entries are given for each of the lowest three spin components because their spacing approaches that of a vibrational spacing and also because the dipole moments appear to differ slightly.
spfit has difficulty treating FeO completely within uncertainties. The lowest three spin-orbit components were reproduced well and the energies and frequencies of the remaining two components are reasonable.
The lines were taken mainly from
(1) T. Kröckertskothen, H. Knöckel, and E. Tiemann, 1987, Mol. Phys. 62, 1031.
Additional lines were taken from
(2) M. D. Allen, L. M. Ziurys, J. M. Brown, 1996, Chem. Phys. Lett. 257, 130.
The partition function takes into account all spin-orbit components. It also includes vibrational corrections in the harmonic approximation. The predictions should be sufficiently accurate for observational purposes.
The dipole moment was determined by
(3) T. C. Steimle, J. Gengler, and P. J. Hodges, 2004, J. Chem. Phys. 121, 12303.