Abstract
Retardation effects on the cross sections for double photoionization of He${(}^{1}$${\mathit{S}}_{0}$) targets are studied at photon energies above 1.0 keV. The dipole asymmetry parameter \ensuremath{\beta} is examined first and the importance of correlation in the final and initial states for this quantity is examined. The effects of retardation on the electron angular distributions are studied and the range of validity of a first-order retardation correction is evaluated. Finally, by using a simple model to calculate the total cross section, the contribution of retardation on the double ionization ${\mathrm{\ensuremath{\sigma}}}^{2+}$ is calculated and it is found that the ratio of double to single photoionization, including retardation, remains the same as when the dipole velocity approximation is employed.
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