Absolute experimental differential and integral electron excitation cross sections for atomic oxygen: 3. The (³P →³D°) transition (989 Å) from 20 to 200 eV with improved values for the (³P →³S°) transition (1304 Å)

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The absolute differential and integral electron excitation cross sections for the atomic oxygen ³P → ³D° transition (989 Å) have been measured at incident energies from 20 to 200 eV. A variable angle and incident energy electron spectrometer coupled to a microwave discharge free supersonic jet expansion source was used for the experiments. The source produced an O/O2 density ratio typically greater than 2.5. The differential cross sections were determined by comparison to computer‐fitted recalculations of the differential cross section for the atomic oxygen ³P →³S° transition (1304 Å). The method used for the computer fit and the results of the fitting calculations are discussed for the ³P →³S° atomic oxygen transition (1304 Å). The O I ³P →³S° (989 Å) differential cross sections vary smoothly with the incident energy and are flatter with scattering angle at the lower incident energies, as expected. The absolute integral cross section is found to agree with the theoretical shape at higher energies. Previous estimates of this cross section from fluorescence data were larger than the present values by approximately a factor of 2. The present lower value appears to provide much better agreement with aurora and dayglow 989 Å emission intensities.