Accurate Rotational Rest Frequencies for Ammonium Ion Isotopologues

We report rest frequencies for rotational transitions of the deuterated ammonium isotopologues NH₃D⁺, ${\text{NH}}_2{\text{D}}_2^{+}$ and $\text{NHD}{\text{D}}_3^{+}$ , measured in a cryogenic ion trap machine. For the symmetric tops NH₃D⁺ and ${\text{NHD}}_3^{+}$ one and three transitions are detected, respectively, and five transitions are detected for the asymmetric top ${\text{NH}}_2{\text{D}}_2^{+}$ . 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.