A study of electron impact excitation of NO: The middle ultraviolet from 170 to 270 nm

Abstract

The electron impact induced fluorescence spectrum of NO at 200 eV has been studied from 170 to 270 nm in a crossed beam experiment. By modeling the laboratory spectrum, including the effects of spin‐orbit splitting of the ground state, we determined the emission cross sections of the γ (A²Σ+ → X²Πr) and ε (D²Σ+ → X²Πr) band systems. The emission cross section of the γ bands includes a large cascade cross‐section contribution from high lying Rydberg states that predominantly populate the υ′ = 0 level of the A²Σ+ state. The ε bands are modeled by excitation rates proportional to Franck‐Condon factors for direct excitation. One feature is tentatively identified as the (1, 0) δ band and permits an estimate of the (C²Πr, υ′ = 1 → X²Πr, υ′ = 0) emission cross section. Previous low‐resolution emission studies of electron impact excitation of the γ bands have not taken into account the heavy spectral blending by the δ, υ, and β′ bands. A major result of this study indicates great care has to be used if NO is taken as a target for relative calibration in the middle ultraviolet (MUV) using the molecular branching ratio technique. The identification of all features in the MUV spectrum allows aeronomers to identify emission features that may arise from electron impact excitation and other chemical processes occurring in the upper atmospheres of the inner planets where NO is a constituent.