Absolute differential and integral electron excitation cross sections for atomic nitrogen. 1. The 4S degree → 3s 4P (λ1200 angstrom) transition from 30 to 100 eV

The absolute direct differential and integral electron excitation cross sections for the atomic nitrogen 4S degree → 3s4P (λ1200 angstrom) transition have been measured at incident energies of 30, 50, and 100 eV. The differential cross sections (DCS) measured versus scattering angle were integrated to give the integral cross section (ICS) as a function of incident energy. The ICS has a maximum value of 66 x 10-18 cm2 near 50 eV. Above 50 eV, the direct excitation cross section is approximately a factor of two lower than fluorescence cross section of Sone and Zipf when the latter is corrected for the recent change in the H2 dissociative excitation reference cross section. This difference suggests that cascade accounts for ∼50% of the fluorescence emission above 50 eV. At 30 eV, the direct cross section is only 1/5 of the fluorescence cross section, suggesting that there is a strong cascade contribution to the fluorescence cross section from forbidden transitions to the 3p 4D degree and 3p 4P degree states just above threshold. The consequences of these new cross section values for aeronomy are discussed