Accurate rotational rest frequencies for ammonium ion isotopologues
  We report rest frequencies for rotational transitions of the deuterated
ammonium isotopologues NH3D+, NH2D2+ and NHD3+, measured in a cryogenic ion
trap machine. For the symmetric tops NH3D+ and NHD3+ one and three transitions
are detected, respectively, and five transitions are detected for the
asymmetric top NH2D2+. While the lowest frequency transition of NH3D+ was
already known in the laboratory and space, this work enables the future radio
astronomical detection of the two other isotopologues.
