The observed transitions were taken from
(1) P. Helminger and W. Gordy, 1969, Phys. Rev. 188, 100;
(2) P. Helminger, F. C. de Lucia, and W. Gordy, 1971, J. Mol. Spectrosc. 39, 94;
(3) L. Fusina, D. di Lonardo, and J. W. C. Johns, 1985, J. Mol. Spectrosc. 112, 211;
(4) S. N. Murzin, 1985, Opt. Spektrosk. 59, 725; and from
(5) L. Fusina and S. N. Murzin, 1994, J. Mol. Spectrosc. 167, 464.
Using two additional higher order centrifugal distortion constants compared to the constants from (5), it was possible to fit the transition frequencies within experimental uncertainties on the average. The dipole moment was taken from
(6) D. di Lonardo and A. Trombetti, 1981, Chem. Phys. Lett. 84, 327.
As for the main isotopomer NH₃, ND₃ tunnels between two equivalent positions. Transitions occur between the symmetric (s) and antisymmetric (a) substates. The a, J = 0 state is higher than the s, J = 0 state by 0.0531 cm^–1 or 1591.6 MHz. In addition, one has to distiguish between A and E state levels with a spin-statistical weight ratio of 11 : 8. The A state levels are described by K = 3n. Moreover, splitting of the A levels into A₁ and A₂ levels with a spin-statistical weight ratio of 10 : 1 has to be taken into account. For K = 3, this splitting can be resolved. For K = 0, the a – s belong to A₁, while the a – s belong to A₂. For K = 6 and higher, the splitting is very small; the two transition frequencies have been merged frequently.
For essentially all of the lines only hyperfine free center frequencies were reported. Since ¹⁴N hyperfine splitting may be resolved for low values of J, a separate calculation with hyperfine splitting is provided for J up to 3.