Nascent rotational distributions of N₂⁺(X ²Σ_g⁺) produced by electron-impact ionization of N₂ in a supersonic beam

Laser-induced fluorescence from nascent N₂⁺(X ²Σ_g⁺) ions produced by electron impact on a N₂ supersonic beam was observed. An analysis of the B ²Σ_u⁺-X ²Σ _g⁺ (0,0) band shows that the rotational state distributions cannot be represented by a single Boltzmann function, higher N″ levels being overpopulated. Experimental and analytical efforts were made to minimize the influence of cascading and relaxation on the rotational distributions. The rotational energy of N₂⁺ (X) thus estimated increases with decreasing electron energy from 2.26±0.16 meV at 300 eV to 4.24±0.27 meV at 25 eV. This trend is explained qualitatively in terms of angular momentum transfer through multipole electron-molecule interactions.