Nitrogen pink afterglow: the mystery continues

This work extends our previous analysis of the nitrogen pink afterglow, by comparing our model predictions with the recently reported measurements of metastable N(P-2) atoms and N-2(a (1)Pi(g)) 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 N-2(A (3)Sigma(+)(u)) and N-2(a' (1)Sigma(-)(u)) metastables. The model predictions significantly overestimate the density of the N-2(a (1)Pi(g)) state, revealing a problem in the description of the singlet kinetics. As singlet N-2(a' (1)Sigma(-)(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.