Large nondipole correlation effects near atomic photoionization thresholds

The parameter that determines the nondipole correction to the angular distribution is calculated for Ar $1s$ and $3s$ subshells in the Hartree-Fock (HF) approximation and taking account of the multielectron correlations, using the random-phase approximation with exchange. In the photoelectron energy range 0--100 eV the parameter, which for s subshells is nonzero at threshold, is found for Ar $3s$ to be strongly affected by multielectron correlations. Results are also presented for He and Be in the HF approximation.