Correlation effects for two-electron systems

For autoionizing levels converging to the N = 2 threshold of helium-like ions with Z = 2-26, the 1/Z expansions of configuration-interaction (CI) energies are compared with the predictions of the S-matrix perturbation theory. The combined data for the expansion coefficients can improve the validity of calculations. Autoionization decay amplitudes and widths are obtained with both the unscreened and screened hydrogen-like orbitals for the free electron, as well as in the first order in continuum Coulomb coupling. Terms of order 1/Z can be evaluated to improve the perturbational data for autoionization amplitudes, involving both screening corrections and the first-order continuum coupling. (author)