Suppressed-Doppler slit jet infrared spectroscopy of astrochemically relevant cations: ν and ν NH stretching modes in NHD.

A suppressed-Doppler (Δν = 180 MHz) infrared spectrum of monodeuterated ammonium ions (NHD) has been obtained for the ν (symmetric) and ν (degenerate) N-H stretch bands via direct absorption high resolution IR laser spectroscopy in a planar slit jet discharge expansion. The ion is efficiently generated by H protonation of NHD in a discharge mixture of H/NHD, with the resulting expansion rapidly cooling the molecular ions into low rotational states. The first high-resolution infrared spectrum of ν is reported, as well as many previously unobserved transitions in the ν rovibrational manifold. Simultaneous observation of both ν and ν permits elucidation of both the vibrational ground and excited state properties of the ion, including rigorous benchmarking of band origins against high-level anharmonic ab initio theory as well as determination of the ν:ν intensity ratio for comparison with bond-dipole model predictions. Ground-state combination differences from this work and earlier studies permit the rotational constants of NHD to be determined to unprecedented accuracy, the results of which support previous laboratory and astronomical assignment of the 1-0 pure rotational transition and should aid future searches for other rotational transitions as well.