Ab-initio calculation of the 2s2 1So-2s3p 3P1 intercombination transition in beryllium-like ions

Transition probabilities of the 2s2 1So-2s3p 1,3P1 intercombination and resonance lines have been calculated for 14 beryllium-like ions in the atomic range 7 ≤ Z ≤ 36. We used multiconfiguration Dirac-Fock wave functions to explore the effects of configuration interaction, of electron rearrangement as well as of relativity in a consistent scheme along the isoelectronic sequence. To show the number of configurations needed in a proper expansion of the wave functions we systematically enlarged the basis from small to large-scale. For the low-Z elements, virtual excitations of the 2s and 3p electrons up to the 5l subshells contribute significantly to the inter-combination probability even though the transition energies remain almost unaffected by excitations beyond the 3l shells. The incomplete orthogonality of the orbital functions due to the rearrangement of the electron density clearly shifts the intercombination rates and appears to be independent of the configuration expansion. This effect decreases, as expected, at higher nuclear charges. Our ab-initio calculation agrees well with a recent measurement by Granzow et al., for ions with 11 ≤ Z ≤ 14. The remarkable deviations near the neutral end of the Be-sequence found in a previous relativistic ab-initio study by Kim et al. have been resolved by increasing the scale of the configuration expansion and by including electron relaxation.