Single and Double Photoionization from Dipole Response Function

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 that is present at high energies when the length gauge is used to characterize the photon 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. {copyright} {ital 1997} {ital The American Physical Society}