Vibrational populations of N₂(A³Σu⁺) in a pure nitrogen glow discharge

The populations of the N 2 (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*10 12 cm -3 , depending on pressure. An interpretation of the experimental data is given using a kinetic model which includes excitation of individual N 2 (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 N 2 (A) and N 2 (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.