Suppressed-Doppler slit jet infrared spectroscopy of astrochemically relevant cations: ν1 and ν4 NH stretching modes in NH3D+
A suppressed-Doppler (Δν = 180 MHz) infrared spectrum of monodeuterated ammonium ions (NH3D+) has been obtained for the ν1 (symmetric) and ν4 (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 H3+ protonation of NH2D in a discharge mixture of H2/NH2D, with the resulting expansion rapidly cooling the molecular ions into low rotational states. The first high-resolution infrared spectrum of ν1 is reported, as well as many previously unobserved transitions in the ν4 rovibrational manifold. Simultaneous observation of both ν1 and ν4 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 ν1:ν4 intensity ratio for comparison with bond-dipole model predictions. Ground-state combination differences from this work and earlier studies permit the rotational constants of NH3D+ to be determined to unprecedented accuracy, the results of which support previous laboratory and astronomical assignment of the 10–00 pure rotational transition and should aid future searches for other rotational transitions as well.
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The Journal of Chemical Physics
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