¥ A ¦ method for including the correlated motion of the electrons in the calculation of single and double ionization of atomic and molecular systems by the absorption of a single photon is described. The correlated § dipole response function is central to the formulation. The single and double photoionization cross § sections of helium are calculated. We show that the method accurately corrects for the error thaẗ is present at high energies when the length gauge is used to characterize the

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... n interaction. The cross sections obtained from the length, velocity, and acceleration gauges are brought into agreement © with each other and with cross sections calculated from the many-body perturbation theory. [S0031-9007 (97) 03175-X] PACS numbers: 32.80.Fb ¤ 3662 ¥ 0031-9007¦ 97 § © 78(19) ¡ ª 3662(3) ¥ $10.00