Fine and hyperfine structure of the N=1-0 transition of ND (X3S-) in vibrational excited states..

The deuterated radical ND was produced in a DC discharge cell cooled at liquid nitrogen temperature. The discharge proved to be vibrationally hot, therefore the transient species could be detected in its vibrational excited states up to v¼6. By scanning in the 431–531 GHz frequency region, several fine-structure components of the N ¼ 1 0 transition in vibrational excited states were observed, each of them showing a complex hyperfine structure. A global analysis, including the measured frequencies and previous submillimetre-wave and infrared data, allowed an accurate determination of the equilibrium spectroscopic parameters of the ND radical including fine and hyperfine constants. A very precise determination of the equilibrium bond length r e was obtained. This value is not consistent with the value reported in the literature from NH data. This incongruity was discussed in terms of the breakdown of the Born–Oppenheimer approximation. In view of the recent detection of ND in a solar-mass protostar [A. Bacmann et al., Astron. Astrophys. 521, L42 (2010)], an extended spectroscopic characterization of this deuterated isotopologue of the NH species may prove useful, considering the large deuterium enhancement observed in molecular clouds.