Optically Forbidden Transition of A'3Δu ← X3Σg− 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'3Δu ← X3Σg− is observed from the measured spectra, and it is found that the peak profile of the A'3Δu ← X3Σg− 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'3Δu ← X3Σg− which are independent of the scattering condition, have been determined for the first time. The peak profiles of the A'3Δu ← X3Σg− determined in this work provide a possibility to realize the spectral decomposition method of unfolding the Herzberg pseudocontinuum proposed by Campbell et al. [Phys. Rev. A 61 (2000) 022706] (atomic and molecular physics)