The experimental lines were taken from
(1) Y. Endo, S. Saito, and E. Hirota, 1983, J. Mol. Spectrosc. 97, 204;
from
(2) M. Kajita, Y. Endo, and E. Hirota, 1987, J. Mol. Spectrosc. 124, 66; and from
(3) L Margulés, E. Herbst, V. Ahrens, F. Lewen, G. Winnewisser, and H. S. P. Müller, 2002, J. Mol. Spectrosc. 211, 211.
The dipole moment is an estimate from several ab initio calculations by
(4) H. L. Woodcock, S. S. Wesolowski, Y. Yamaguchi, and H. F. Schaefer, III, 2001, J. Phys. Chem. A 105, 5037.
At low temperatures, it may be necessary to discern between ortho-PH₂ and para-PH₂. The ortho states are described by K_a + K_c even, the para states by K_a + K_c odd. There are three times as many levels for ortho-PH₂ than there are for para-PH₂. Thus, for transitions with unresolved ¹H hyperfine splitting the nuclear spin-weight ratio is 3 : 1 between ortho-PH₂ and para-PH₂. However, for transitions with resolved ¹H hyperfine splitting no non-trivial spin-statistics have to be considered. The N_KaKc = 1₀₁; n, F = 5, 1 (or J, F₁ = 1.5, 1.0) is the lowest para state. It is 12.2771 cm^–1 above ground.
The fourth quantum number is an aggregate spin-number it can be decoded with the information provided in the lower part of the ph2.out file or in the upper part of the ph2.fit file.