Sub-Doppler slit jet infrared spectroscopy of astrochemically relevant cations: The NH stretching mode in NDH.

High-resolution rotationally resolved spectra of the N-H stretch vibrational mode ( ) of jet-cooled NDH ions are collected and analyzed in a sub-Doppler slit-jet infrared spectrometer. The isotopomeric ammonium ions are generated by proton transfer from H to ND in a discharge of an ND/H gas mixture, whereby the slit jet expansion cools the nascent NDH ions into lower rotational states. Rotational assignments are confirmed by four-line combination differences that agree to within the spectrometer precision (9 MHz). Based on precision two-line ground-state combination differences and a symmetric top Hamiltonian, the , , and rotational constants for the ground vibrational state of NDH are determined with high precision for the first time. Approximate rotational constants for the excited state are also determined, with a band origin at 3316.8425(19) cm and in remarkable (∼0.1 cm) agreement with high level anharmonic theoretical predictions by Guo and co-workers [J. Phys. Chem. A, , 2185 (2016)]. Our results allow us to predict several low- pure rotational transitions of NDH, which we hope will support future studies of this important ion in laboratory and astronomical rotational spectroscopy.