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 -ampbell ？t et al. [Phys. Rev. A 61 （2000） 022706]