Absolute differential and integral electron excitation cross sections for atomic oxygen 7. The 3P → 1D and 3P → 1S transitions from 4.0 to 30 eV

Abstract

We report the absolute differential and integral cross sections of the forbidden atomic oxygen 3P → 1D( 1.97 eV), and 3P → 1S( 4.19 eV), transitions for incident electron impact energies of 4.0 to 30 eV. The 3P → 1D cross section was measured over the entire energy range while the 3P → 1S transition was measured above 7 eV. The differential cross section (DCS) for the 3P → 1D transition is strongly backward peaked at all incident energies, in agreement with theoretical calculations. The DCS for the 3P → 1S transition varies considerably in shape over the energy range 7–30 eV. At the lower energies, the transition shows an axially forbidden shape (no intensitiy in the forward or backward direction) in agreement with the Thomas and Nesbet (1975) calculations. At higher energies (20–30 eV), the DCS has a remarkable drop near 70°. The agreement between the DCS measured here and the previous measurements of Shyn and Sharp (1986) and Shyn et al. (1986) varies from fair to poor at different incident energies. The integral cross section (ICS) for the 3P → 1D transition agrees within experimental error with the measurements of Shyn and Sharp (1986) and the theoretical calculations of Henry et al. (1971) at 30 eV but is larger than the previous measurements by a factor of approximately 1.3 down to 15 eV and a factor of 2 near the 5 eV peak. The low‐energy peak in the ICS is shown to be between 5 and 7 eV incident energy and the 4 eV experimental measurement is definitely below the peak. No pronouced peak is observed for the 3P → 1S transition. The ICS is in good agreement with the calculations at 30 eV but is larger than the calculations by a factor of approximately 1.4 at lower energies down to 7 eV. The present results are systematically smaller in magnitude than those of Shyn et al. (1986) over the entire energy range studied. The difference varies from a factor of 1.3 at 10 eV to 2.5 at 30 eV. There is no pronounced peak in the 3P → 1S ICS, in agreement with the theoretical calculations.