Terahertz spectroscopy of N$^{18}$O and isotopic invariant fit of
  several nitric oxide isotopologs
  A tunable far-infrared laser sideband spectrometer was used to investigate a
nitric oxide sample enriched in 18O between 0.99 and 4.75 THz. Regular,
electric dipole transitions were recorded between 0.99 and 2.52 THz, while
magnetic dipole transitions between the 2Pi(1/2) and 2Pi(3/2) spin-ladders were
recorded between 3.71 and 4.75 THz. These data were combined with lower
frequency data of N(18)$O (unlabeled atoms refer to (14)N and (16)O,
respectively), with rotational data of NO, (15)NO, N(17)O, and (15)N(18)O, and
with heterodyne infrared data of NO to be subjected to one isotopic invariant
fit. Rotational, fine and hyperfine structure parameters were determined along
with vibrational, rotational, and Born-Oppenheimer breakdown corrections. The
resulting spectroscopic parameters permit prediction of rotational spectra
suitable for the identification of various nitric oxide isotopologs especially
in the interstellar medium by means of rotational spectroscopy.
