Isotopes of ammonia

Ammonia is the first polyatomic molecule detected in the interstellar space. Along with hydrogen, helium, methane and water, it is abundant in the atmospheres of Jupiter, Saturn, Uranus, and Neptune, in the intestellar medium, in comets and meteorites. However, due to heavy predissociation and spectral congestion, only few excited electronic states of ammonia have been observed and investigated. Using a monochromatic VUV beam generated by two-photon resonant sum frequency mixing in a Mg-Kr mixture, we have investigated the electronic transitions of NH₃ and ND₃ in the 64500-73000 cm^-1 region by detecting the fluorescence from the predissociation fragments (mainly due to the decay from the excited electronic A-states of NH₂ or ND₂) or by detecting the ions produced via VUV+visible resonance enhanced multiphoton ionization (REMPI). Spectral congestion is removed with the use of a pulsed molecular beam which greatly simplifies the spectral analysis and enables the observation of weak vibronic bands. Different spectral features are identified, rotationally resolved vibronic bands were analysed and rotational constants are obtained. Based on the high resolution spectra we were able to reassign the symmetry of the D electronic state to E' which was misassigned before.

Quick-look spectrum of NH₃

Quick-look spectrum of ND₃