Linear and nonlinear screening effects in two-photon ionization of xenon

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Abstract
We apply the random-phase approximation to two-photon ionization. We develop a formalism which, by means of screened electron-photon interactions, includes the nonlinear response and, in particular, double-excitation processes. The theory is illustrated by numerical calculations of the two-photon ionization cross section of the external (5p and 5s) shell of xenon, over a large energy range: from the two-photon ionization threshold, in the discrete resonance region, and above the one-photon ionization threshold. We show the influence of different many-electron effects, for example, ground-state and final-state correlation, and linear and nonlinear screening. Compared with an independent-electron picture, screening effects lead to a large reduction of the ionization cross section at low photon energy and in contrast, to an important enhancement at higher photon energy, due to the influence of double-excitation and -ionization processes.