In very pure nitrogen, at high current and pressure, the relative intensities of the bands emitted by the positive column are reversed as compared with those emitted in the conventional discharge. The second positive system, although ordinarily dominating, may be completely suppressed and the N<sub>2</sub><sup>+</sup> bands strongly enhanced. We attribute the reversal to excitation and ionization in steps via metastable levels. Their low ionization potential limits the energy of the electrons. Possibly the reversal is enhanced by a selection rule. This process, excluding the second positive bands, is contrasted with processes exciting the same bands as the only visible system, namely, the blue afterglow and recombination of atoms on a copper surface. In these experiments, the first positive bands, although not observed, are inevitably excited by cascading but fail to exhibit strong vibration. This limits them to the infrared, visually unobservable range.
O. OLDENBERG, "Reversal of Relative Intensities of Band Systems of Nitrogen," J. Opt. Soc. Am. 61, 1098-1100 (1971)