Vibrational populations of N₂(A³Σ_u⁺) in a pure nitrogen glow discharge

Abstract

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.