Photoionization accompanied by excitation at intermediate photon energies

We calculate the photoionization-with-excitation to photoionization ratios Rnl and Rn = Σl Rnl for atomic helium and positive heliumlike ions at intermediate values of the photon energies. The final-state interactions between the electrons are included in the lowest order of their Sommerfeld parameter. This enables us, in contrast to purely numerical calculations, to investigate the roles of various mechanisms contributing beyond the high-energy limit. The system of two bound electrons is described by the functions obtained by the correlation function hyperspherical harmonic method. For the case of heliumlike ions we present the high-energy limits as a power expansion in the inverse charge of the nucleus. We analyze the contribution of excitation of states with nonzero orbital momenta to the ratios Rn. In the case of helium our results for Rn are in good agreement with those of experiments and of previous calculations.