Vibrational populations of N₂(A ³Σ_u⁺) in a pure nitrogen glow discharge

The populations of the N₂(A ³ Sigma _u⁺, nu =0-5) states in a pure nitrogen glow discharge have been determined from measurements of the absorption of the first positive system bands for gas pressures in the range 0.1-2 Torr and discharge current densities up to about 30 mA cm^-2. The observed vibrational distributions are highly non-Boltzmann and an inversion of population between the levels nu =2 and nu =3 was found. The total population in the nu =0-5 levels saturates with increasing current, the saturated population densities ranging from about 3 to 6*10¹² cm^-3, depending on pressure. An interpretation of the experimental data is given using a kinetic model which includes excitation of individual N₂(A, nu ) vibrational levels by electron impact with ground-state molecules and by cascading from the B ³ Pi _g to the A state, diffusion to the wall, quenching by nitrogen atoms, metastable-metastable energy pooling, vibration-vibration energy transfer between N₂(A) and N₂(X) molecules, and collisionally induced transitions between the B ³ Pi _g and A ³ Sigma _u⁺ states. The predictions are in good agreement with the measurements. However the observed inversion of population between the nu =2 and nu =3 levels cannot be explained by the above mechanisms. It is suggested that is due to vibrationally dependent quenching.