Resonance energies, oscillator strengths and autoionization widths of the ls-2p excitations from O IV low-lying states

The excitation energies and oscillator strengths of the 1s-2p transitions from O iv low-lying terms belonging to the configurations 2s(2)2p, 2s2p(2) and 2p(3) are calculated using configuration interaction (CI) wavefunctions. Reasonably good agreement is obtained between the length and velocity forms. To verify the correctness of the results, these data are further obtained by treating the 1s-2p excitations as resonances occurring in photoionization processes. The photoionization cross sections are computed using the close-coupling scheme implemented by the R-matrix method. The resonance energies and autoionization widths of the 1s-2p excited states are determined by analysing the resonances exhibited by the photoionization cross sections. The results show that the resonance energies obtained by the R-matrix method are systematically lower than those by the CI scheme by 0.1-0.2 Ryd. This shows that a strong Cl effect exists between the K-shell excited and continuum states. This effect can also produce redistribution of the oscillator strengths, especially for the weak transitions.