Laboratory rotational ground state transitions of NH3D+ and CF+

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 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 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 photon-dominated region (PDR).