2016A&A...593A..56S

Aims. This paper reports accurate laboratory frequencies of the rotational ground state transitions of two astronomically relevant molecular ions, NH₃D⁺ and CF⁺.
Methods. Spectra in the millimetre-wave band were recorded by the method of rotational state-selective attachment of He atoms to the molecular ions stored and cooled in a cryogenic ion trap held at 4K. The lowest rotational transition in the A state (ortho state) of NH₃D⁺ (J_K=1₀-0₀), and the two hyperfine components of the ground state transition of CF⁺ (J=1-0) were measured with a relative precision better than 10^–7.
Results. For both target ions, the experimental transition frequencies agree with recent observations of the same lines in different astronomical environments. In the case of NH₃D⁺ the high-accuracy laboratory measurements lend support to its tentative identification in the interstellar medium. For CF⁺ the experimentally determined hyperfine splitting confirms previous quantum-chemical calculations and the intrinsic spectroscopic nature of a double-peaked line profile observed in the J=1-0 transition towards the Horsehead photon-dominated region (PDR).