Diazenylium

[1] Its 1–0 rotational transition occurs at 93.174 GHz, a region of the spectrum where Earth's atmosphere is transparent[2] and it has a significant optical depth in both cold and warm clouds[3] so it is relatively easy to observe with ground-based observatories.

This was done using a combination of ab initio molecular calculations and comparison of similar molecules, such as N2, CO, HCN, HNC, and HCO+, which are all isoelectronic to N2H+.

[6] Just a year later, Thaddeus and Turner observed the same transition in the Orion molecular cloud 2 (OMC-2) using the same telescope, but this time they integrated for 26 hours, which resulted in a resolution that was good enough to distinguish the smaller hyperfine components.

The first laboratory spectrum of N2H+ was of the 1–0 rotational band in the ground vibrational level, the same microwave transition that astronomers had recently discovered in space.

[11] Ten years later, Owrutsky et al. performed vibrational spectroscopy of N2H+ by observing the plasma created by a discharge of a mixture nitrogen, hydrogen, and argon gas using a color center laser.

Given the selection rule ΔJ = ±1, the calculated rotational energy levels, along with their percent population at 30 kelvins, can be plotted.

[15] The calculated rates here were for early time (316,000 years) and a temperature of 20 kelvins, which are typical conditions for a relatively young molecular cloud.

[14] Although the actual electron density in so-called "dense clouds" is quite low, the destruction of N2H+ is governed mostly by dissociative recombination.