Nitrogen pink afterglow: the mystery continues

Abstract

This work extends our previous analysis of the nitrogen pink afterglow, by comparing our model predictions with the recently reported measurements of metastable N(2P) atoms and N2(a 1Πg) molecules. It is shown that both species reveal the presence of a characteristic maximum on their populations, occurring downstream from the discharge after an initial stage of decrease. Such behavior is a consequence of the V-V pumping-up mechanism taking place during the relaxation in the afterglow, which is followed by V-E transfers that create locally N2(A 3Σ+u) and N2(a′ 1Σ−u) metastables. The model predictions significantly overestimate the density of the N2(a 1Πg) state, revealing a problem in the description of the singlet kinetics. As singlet N2(a′ 1Σ−u) metastables play a crucial role in nitrogen ionization, the new results imply that the ionization mechanisms in the afterglow may have to be reviewed.