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 NH_{3}, ND_{3} 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 spinstatistical weight ratio of 11 : 8.
The A state levels are described by K = 3n.
Moreover, splitting of the A levels into A_{1}
and A_{2} levels with a spinstatistical 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 A_{1}, while the a – s
belong to A_{2}.
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 ^{14}N hyperfine splitting
may be resolved for low values of J, a separate
calculation with hyperfine splitting is provided for J up to 3.
