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Protonated molecular nitrogen (protonated dinitrogen), N2H+, is used by radio astronomers and astrochemists as a tracer of dense gas in molecular clouds, since its large electric dipole moment requires relatively high densities to collisionally excite the pure rotational transitions at millimeter wavelengths. Since the presumed reservoir of most nitrogen in these clouds, molecular nitrogen (N2), has no easily accessible transitions, N2H+ is an important indicator of the nitrogen chemistry in these regions. It is particularly useful, since it is one of the last gas phase species to freeze out onto the interstellar dust grains at the center of cold clouds.


The ion N2H+ was identified in interstellar molecular clouds by means of the characteristic hyperfine splitting pattern of the fundamental rotational transition (J = 1−0) at 93 GHz (Turner 1974), 2 years before this transition was measured in the laboratory, providing an interesting ...