Submillimeter and Far-infrared Spectroscopy of Monodeuterated Amidogen Radical (NHD): Improved Rest Frequencies for Astrophysical Observations

Observations of ammonia in interstellar environments have revealed high levels of deuteration, and all its D-containing variants, including ND3, have been detected in cold prestellar cores and around young protostars. The observation of these deuterated isotopologues is very useful for elucidating 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 prebiotic interest, but relatively little is known about their D-bearing isotopologues. To date, only ND has been detected in interstellar gas. To aid the identification of further deuterated nitrogen radicals, we have thoroughly reinvestigated the rotational spectrum of NHD by employing two different instruments: a frequency-modulation submillimeter spectrometer operating in the THz region and a synchrotron-based Fourier-transform infrared spectrometer operating in the 50-240 cm-1 frequency range. NHD was produced in a plasma of NH3 and D2. A wide range of rotational energy levels have 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 data from the literature has provided a comprehensive set of very accurate spectroscopic parameters. A highly reliable line catalog has been generated to assist archival data searches and future astronomical observations of NHD at submillimeter and THz regimes.