The ammonium cation, NH4+, is among the most stable gas phase cations because of the very large proton affinity of ammonia, NH3. However, NH4+ itself can not be detected by radio astronomy because the spherical symmetry precludes the molecule from having a permanent electric dipole moment. In contrast, the separation of the charge from the center of mass causes the mono-deuterated isotopolog to have a comparatively large dipole moment of about 0.26 D.
J. Cernicharo, B. Tercero, A. Fuente, J. L. Domenech, M. Cueto, E. Carrasco,
V. J. Herrero, I. Tanarro, N. Marcelino, E. Roueff, M. Gerin, and J. C. Pearson
Detection of the Ammonium Ion in Space
Astrophys. J. 771, Art. No. L10 (2013).
The JK = 10 00 transition of NH3D+ at a derived rest frequency of 262816.73 (10) MHz was detected with the IRAM 30 m telescope. The line was detected with good signal-to-noise ratio in the star-forming region Orion-IRc2 in the course of a molecular line survey and just barely in the dark cloud B1-bS in subsequent dedicated observations. The detection is probably fairly secure even if the laboratory rest frequency was only determined with infared accuracy, and even though Orion-IRc2 is rather line rich.