Absolute experimental differential and integral electron excitation cross sections for atomic oxygen: 4. The (³P → 3s″³P°), (³P → 2s 2p5 ³P°), (³P → 4d′ ³P°) Autoionizing Transitions (878, 792, and 770 Å) and five members of the (³P → nd ³D°) Rydberg series (1027 Å)

Abstract

Absolute differential and integral direct electron excitation cross sections have been measured for the three most intense autoionizing ³P → ³P° transitions in atomic oxygen (878, 792, and 770 Å) and the first five members of the ³D° Rydberg series whose origin is the 1027 Å transition. Incident electron energies from 30 to 200 eV were used. Absolute values for the differential cross sections were calculated using the previously measured atomic oxygen ³P → ³S° (1304 Å) cross section of Vaughan and Doering (1987) as a secondary standard. The cross sections were much more strongly forward peaked at the higher laboratory incident energies than at low energies. The integral cross sections for the 878‐ and 792 Å transitions were identical within experimental error, both in shape and absolute magnitude. The peak values were 1.1‐1.4 × 10−17 cm² at the maximum near 50 eV, about a factor of 3 lower than the total cross section deduced by Zipf and Kao (1986) from emission experiments. The 770 Å transition was approximately a factor of 4 weaker with the same shape. The ³D° Rydberg series cross sections decrease uniformly with increasing principal quantum number as expected. The peak of the 1027 Å origin transition is approximately 3.5 × 10−18 cm² at 50 eV, in satisfactory agreement with the emission cross section of Zipf and Erdman (1985) The present cross sections for the higher energy states suggest that the autoionizing transitions are a significant source of O+(4S) ions in the thermosphere, but not as strong a source as has been inferred on the basis of previous emission measurements.