Optically Forbidden Transition of A'³Δ_u ← X³Σ_g^-in Oxygen

The electron energy loss spectra for the valence-shell excitations of oxygen are measured at an incident electron energy of 2500 eV and at scattering angles from 4^° to 8.5^°. The dipole−forbidden and octupole-allowed transition of the A'³Δ_u ←X³Σ_g^- is observed from the measured spectra, and it is found that the peak profile of the A'³Δ_u ←X³Σ_g⁻ excitation can be well represented by a Gaussin function. The energy position of 6.007±0.008 eV and Franck−Condon linewidth of 1.312±0.020 eV of the A'³Δ_u ←X³Σ_g⁻, which are independent of the scattering condition, have been determined for the first time. The peak profiles of the A'³Δ_u ←X³Σ_g⁻ determined in this work provide a possibility to realize the spectral decomposition method of unfolding the Herzberg pseudocontinuum proposed by Campbell ıt et al. Phys. Rev. A 61 (2000) 022706