Accurate Rotational Rest Frequencies for Ammonium Ion Isotopologues.

We report rest frequencies for rotational transitions of the deuterated ammonium isotopologues NH3D+, NH 2 D 2 + and NHD D 3 + , measured in a cryogenic ion trap machine. For the symmetric tops NH3D+ and NHD 3 + one and three transitions are detected, respectively, and five transitions are detected for the asymmetric top NH 2 D 2 + . 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.