Sub-Doppler slit jet infrared spectroscopy of astrochemically relevant cations: Symmetric (ν) and antisymmetric (ν) NH stretching modes in NDH.

Sub-Doppler infrared rovibrational transitions in the symmetric (v) and antisymmetric (v) NH stretch modes of the isotopomerically substituted NDH ammonium cation are reported for the first time in a slit jet discharge supersonic expansion spectrometer. The partially H/D substituted cation is generated by selective isotopic exchange of ND with HO to form NHD, followed by protonation with H formed in the NHD/H/Ne slit-jet discharge expansion environment. Rotational assignment for NDH is confirmed rigorously by four line ground state combination differences, which agree to be within the sub-Doppler precision in the slit jet (∼9 MHz). Observation of both b-type (ν) and c-type (ν) bands enables high precision determination of the ground and vibrationally excited state rotational constants. From an asymmetric top Watson Hamiltonian analysis, the ground state constants are found to be A″ = 4.856 75(4) cm, B″ = 3.968 29(4) cm, and C″ = 3.446 67(6) cm, with band origins at 3297.5440(1) and 3337.9050(1) cm for the v and v modes, respectively. This work permits prediction of precision microwave/mm-wave transitions, which should be invaluable in facilitating ongoing spectroscopic searches for partially deuterated ammonium cations in interstellar clouds and star-forming regions of the interstellar medium.