Cross section for the production of N₂ ⁺ (X²Σ_g ⁺) ions by electron impact on N₂

A combination of electron scattering and laser-induced fluorescence (LIF) techniques has been employed in the direct experimental determination of the N₂⁺ (X²Σ_g⁺) ionizati on cross section as a function of electron energy from threshold to 200 eV. Electron impact on N₂ produces N₂⁺ ground-state ions that are detected by pumping the X ²Σ_g⁺ → B ²Σ_u⁺ + (0,0) vibrational transition at 391 nm with a tunable dye laser and detecting the subsequent LIF of the B ²Σ_u⁺ → X ²Σ_g⁺ (0,1) vibrational transition at 428 nm. LlF spectra obtained at different electron energies yield the relative N₂⁺(X) cross section which is put on an absolute scale by normalization to the absolute cross section value of Doering and Yang [J. Geophys. Res. 102 (1996) 9683] obtained from electron-electron coincidence(e,2e) experiments at 100 eV.