Context :

Aims : This paper reports accurate laboratory frequencies of the rotational ground state transitions of two astronomically relevant molecular ions , NH _ { 3 } D ^ { + } and CF ^ { + } .

Methods : Spectra in the millimeter-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 4 K. The lowest rotational transition in the A state ( ortho state ) of NH _ { 3 } D ^ { + } ( J _ { K } = 1 _ { 0 } -0 _ { 0 } ) , 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 NH _ { 3 } D ^ { + } 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 PDR .

Conclusions :