High resolution rotation-inversion spectroscopy on doubly deuterated ammonia, ND2H, up to 2.6Â THz

Frequency multiplication of phase locked, high power backward wave oscillators (BWO) permitted to record high accuracy spectra of ND2H up to frequencies of 2.6 THz. Novel superlattice (SL) devices were used as key-element to generate higher order harmonics for the first time in spectroscopic applications. In total, 240 measured ground state transitions of the pure rotation and inversion-rotation spectra of ND2H were measured with accuracies of several kilohertz in the frequency region between 0.08 and 2.58 THz. Energy levels up to quantum numbers J=18 and Ka=9 have been accessed. A greatly extended experimental dataset was obtained with significantly improved accuracies. Two sets of molecular parameters have been determined from which predictions can be generated that can be regarded as a reliable basis for future astronomical high resolution observations throughout the microwave to terahertz regions. These predictions are available in the Cologne database for molecular spectroscopy (www.cdms.de).