A reexamination of important N2 cross sections by electron impact with application to the dayglow: The Lyman‐Birge‐Hopfield Band System and N I (119.99 nm)

Abstract

The far ultraviolet emission spectrum (120 to 210 nm) of electron‐excited N2 has been obtained in a crossed‐beam laboratory experiment. The cross section of the Lyman‐Birge‐Hopfield (LBH) band system (a¹Πg → X¹Σg+) has been remeasured using experimental techniques we have previously developed for this metastable transition. The improved laboratory data set for the a¹Πg state allows a determination of the excitation, emission, and predissociation cross section from threshold to 200 eV for use in planetary atmosphere models of the dayglow and aurora. An analytic fit to the experimental cross section allows accurate estimates to arbitrarily high excitation energy. The close agreement in both energy dependence and absolute cross section values between the emission measurements, presented here, and published electron scattering results shows cascade is small (<5%). The total excitation cross section for the N2 a¹Πg state is estimated to be 6.22 ± 1.37×10−18 cm² at 100 eV. The absence of emission bands for υ′ ≥ 7 suggests the predissociation yield is unity. The excitation function of each vibrational level is found to have the same shape to within 5%. In the low‐energy region, ε < 20 eV, differences in excitation threshold lead to a significant departure of the relative vibrational cross sections from the Franck‐Condon distribution. Thus the relative LBH vibrational population distribution in a planetary dayglow or aurora is affected by the energy distribution of the electron flux; and we show that atmospheric models need to include this threshold effect. The N I (119.99 nm) cross section has also been remeasured and found to be 3.48 ± 0.77×10−18 cm² at 200 eV on the basis of a comparison with Lyman α emission from dissociative excitation of H2.