Submillimeter and Far-infrared Spectroscopy of Monodeuterated Amidogen Radical (NHD): Improved Rest Frequencies for Astrophysical Observations
Résumé
Observations of ammonia in interstellar environments have revealed high levels of deuteration. All its D-containing variants, NH2D, ND2H, and even ND3 , have been detected in cold prestellar cores and around young protostars. The observation of these deuterated isotopologues is very useful to elucidate the chemical and physical processes taking place during the very early stages of star formation, as the abundance of deuterated molecules is highly enhanced in dense and cold gas. Nitrogen hydride radicals are key species lying at the very beginning of the reaction pathway leading to the formation of NH3 and organic molecules of pre-biotic interest, but relatively little information is known about their D-bearing isotopologues. To date, only ND has been detected in the interstellar gas. To aid the identification of further deuterated nitrogen radicals, we have thoroughly re-investigated the rotational spectrum of NHD, the monodeuterated isotopologue of the amidogen radical NH2. The spectrum has been recorded with two different instruments: a frequency-modulation submillimetre spectrometer operating in the THz region and a synchrotron-based Fourier Transform infrared spectrometer operating in the 50-240 cm-1 frequency range. In both cases, NHD was produced in a plasma of NH3 and D2. A wide range of rotational energy levels has been probed thanks to the observation of high N (up to 15) and high K a (up to 9) transitions. A global analysis including our new data and those already available in the literature has been performed yielding a comprehensive set of highly accurate spectroscopic parameters. A highly reliable line catalogue has been generated to assist archival data searches and future astronomical observations of NHD at submillimetre and THz regimes.
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