Pink Afterglow of Nitrogen

Abstract

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 (B³Π_g→A³Σ⁺_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⁺_u→X²Σ⁺_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 detected^4,5 is vibrationally excited, ground state nitrogen—N₂†; from Bass's data, one can estimate T_vib∼10⁴ 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 T_vib 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 T_kin). 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.