Many-electron effects in multiphoton ionization: Screening effects in single-electron ionization

Abstract

We study the influence of many-electron effects in multiphoton ionization within the framework of diagrammatic many-body perturbation theory. We renormalize the electron-dipole coupling by summing to infinite order both many-electron interactions using the random-phase approximation and higher-order intensity terms. We introduce an effective intensity, which takes into account the screening of the field by the electrons and represents the intensity really seen by an electron. The theory is applied to a calculation of the two-photon one-electron ionization rate of helium in the weak-field limit, using a local-density approximation one-electron basis set. The influence of many-electron effects strongly depends on the field frequency. The two-photon ionization rate of helium is lowered at low frequency (by a factor up to 1.4) and increased at high frequency when the photon energy is above the ionization threshold. Finally, the importance of many-electron effects in multiphoton ionization (e.g., regarding inner-shell ionization) is qualitatively discussed in connection with experiments.