Pink Afterglow of Nitrogen


NITROGEN which has passed through a discharge contains free atoms: their recombination gives rise to the long-lived, yellow, Lewis–Rayleigh afterglow (LRA)1 which is composed mainly of first positive bands (B3ΠgA3Σ+u) with v′ ≤ 12 (the dissociation limit). Microwave discharges in pure nitrogen also give a short-lived, pink afterglow (PA)2,3. In this case, the first positive bands are excited to much higher vibrational levels and there is strong emission of the first negative bands (N+2, B2Σ+uX2Σ+g). A remarkable feature is the dark space separating the discharge and the PA in which only the weaker LRA is visible. The only additional energy carrier that has been detected4,5 is vibrationally excited, ground state nitrogen—N2†; from Bass's data, one can estimate Tvib104 K. It is not known whether vibration–vibration exchange will lead to a Boltzmann energy distribution in the PA (10 ms after the discharge), so the term Tvib is used loosely, but it is certain that vibrational energy will not equilibrate rapidly with translation and rotation (the “gas kinetic” temperature will here be called Tkin). On the other hand, equilibration between vibrational energy of N2 and kinetic energy of the electrons (reaction (2)) is very fast6 and average electron energies of 4 eV have been found in the PA7.

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BROCKLEHURST, B., NICHOLLS, R. Pink Afterglow of Nitrogen. Nature 223, 824–825 (1969).

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