Absolute cross sections for electron-impact excitation of N2+

Crossed beams of N₂⁺ and electrons were used to measure the absolute cross sections for the excitation, $N_2^{{}^{+}}(X^2{\Sigma }_g^{+},v=0)+e\to N_2^{{}^{+}}(B^2{\Sigma }_u^{+},v=0)+e$, over an electron energy range from below threshold (3.17 eV) to 91 eV. Absolute emissions of the 391.4-nm band were measured from impact of electrons on N₂⁺ ions in various state mixtures. Corrections were made to the data to account for state mixtures and other effects to obtain the cross section for the above process. The cross section falls from its finite threshold value of 3.0 × 10⁻¹⁶ to 0.30 × 10⁻¹⁶ cm² at 91 eV. The present values are more than an order of magnitude smaller than either the 1968 values of Lee and Carleton or the 1973 value of Daschenko et al. At threshold the Gaunt-factor formula of Seaton predicts a value only 45% of that measured, but at the highest energy, the predictions of the Seaton formula have converged to within 20% of the present values of the cross section. Rate coefficients calculated from the present measurements are consistent with the recent rate-coefficient measurements of McLean et al. Total uncertainty at high confidence is about 18%, taken as the quadrature sum of random uncertainty (15% at 98% confidence level) with systematic uncertainties (about 8% at high confidence). The analysis leading to interpretation of the emission cross section in terms of an excitation cross section between specific states is subject to uncertainties which are not well defined, and are not included in the stated uncertainty.