Many-electron character of two-photon above-threshold ionization of Ar

The absolute generalized cross sections and angular distribution parameters of photoelectrons for the two-photon above-threshold 3p ionization of Ar were calculated in the exciting photon energy range from 15.76 to 36 eV. The correlation function technique developed earlier was extended for the case when an intermediate-state function is of a continuum-type. We show that two-photon ionization of Ar near the 3p(4) threshold to a large extent is determined by the (3p -> epsilon d)(2) two-photon absorption via the giant resonance. This many-electron correlation causes (i) an increase of the photoionization cross sections by more than a factor of 3; and (ii) the appearance of resonances in the exciting-photon energy range of the doubly excited states. The predictions are supported by a good agreement between length and velocity results obtained after taking into account the higher-order perturbation theory corrections.