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diazenylium
Diazenylium is the chemical N2H+, an inorganic cation that was one of the first ions to be observed in interstellar clouds. Since then, it has been observed for in several different types of interstellar environments, observations that have several different scientific uses. It gives astronomers information about the fractional ionization of gas clouds, the chemistry that happens within those clouds, and it is often used as a tracer for molecules that are not as easily detected (such as N2).〔(P. Caselli, P.C. Myers, and P. Thaddeus, ApJL, 455: L77 (1995) )〕 Its 1-0 rotational transition occurs at 93.174 GHz, a region of the spectrum where Earth's atmosphere is transparent〔(CSO Atmospheric Transmission Interactive Plotter )〕 and it has a significant optical depth in both cold and warm clouds〔(L. Pirogov, I. Zinchenko, P. Caselli, L.E.B. Johansson and P. C. Myers, A&A, 405: 639-654 (2003) )〕 so it is relatively easy to observe with ground-based observatories. The results of N2H+ observations can be used not only for determining the chemistry of interstellar clouds, but also for mapping the density and velocity profiles of these clouds.
== Astronomical detections ==
N2H+ was first observed in 1974 by B.E. Turner. He observed a previously unidentified triplet at 93.174 GHz using the NRAO 11-meter telescope.〔(B. Turner, ApJ, 193: L83 (1974) )〕 Immediately after this initial observation, Green et al. identified the triplet as the 1-0 rotational transition of N2H+. 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+. Based on these calculations, the observed rotational transition would be expected to have seven hyperfine components, but only three of these were observed, since the telescope's resolution was insufficient to distinguish the peaks caused by the hyperfine splitting of the inner Nitrogen atom.〔(S. Green, J. Montgomery, and P. Thaddeus, ApJ, 193: L89 (1974) )〕 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.〔(P. Thaddeus and B.E. Turner, ApJ, 201: L25-L26 (1975) )〕
Over the past three decades, N2H+ has been observed quite frequently, and the 1-0 rotational band is almost exclusively the one that astronomers look for. In 1995, the hyperfine structure of this septuplet was observed with an absolute precision of ~7 kHz, which was good enough to determine its molecular constants with an order of magnitude better precision than was possible in the laboratory.〔(P. Caselli, P. Myers, and P. Thaddeus, ApJL, 455: L77 (1995) )〕 This observation was done toward L1512 using the 37-meter NEROC Haystack Telescope. In the same year, Sage et al. observed the 1-0 transition of N2H+ in seven out of the nine nearby galaxies that they observed with the NRAO 12-meter telescope at Kitt Peak.〔(L. Sage and L. Ziurys, ApJ, 447: 625 (1995) )〕 N2H+ was one of the first few molecular ions to be observed in other galaxies, and its observation helped to show that the chemistry in other galaxies is quite similar to that which we see in our own galaxy.
N2H+ is most often observed in dense molecular clouds, where it has proven useful as one of the last molecules to freeze out onto dust grains as the density of the cloud increases toward the center. In 2002, Bergin et al. did a spatial survey of dense cores to see just how far toward the center N2H+ could be observed and found that its abundance drops by at least two orders of magnitude when one moves from the outer edge of the core to the center. This showed that even N2H+ is not an ideal tracer for the chemistry of dense pre-stellar cores, and concluded that H2D+ may be the only good molecular probe of the innermost regions of pre-stellar cores.
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