We discuss the role of many-electron interactions in multiphoton multielectron ionization of heavy atoms and try to indicate the relative importance of collective effects and screening, relaxation and shakeup, and Fermi sea correlations. The purpose of the work is to look for approximations in which the photoemission processes have been renormalized in consistent ways with respect to the field and the electron–electron interaction. This involves effective shift and broadening of energy levels, effective matrix elements for ionization by the field, effective field-induced density of states for the photoelectron, etc. In an application, we consider the low-intensity limit and discuss two-photon one-electron ionization of the 5p shell of Xe using a zero-field atomic basis set. This calculation demonstrates important ways in which collective effects may enter, and it also represents the first reported calculation of its kind. Finally, we apply the scheme for dressing the many-electron processes to a discussion of time scales in two-step double ionization, introducing the laser pulse duration in an intuitive way.
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