Methods for the calculation of photoionization cross sections using the extended Koopmans' theorem

Abstract

A new approach to the calculation of photoionization cross sections for atoms and molecules is described. The procedure requires for input only a wave function for the parent initial state, obviating the necessity of a separate calculation on the final bound state by making use of the recently developed Extended Koopmans' Theorem. Explicit formulas for the photoionization cross sections, in terms of the one- and two-particle reduced density matrices of the initial state wave function, have been obtained at several levels of sophistication, including the sudden approximation, an orbital model for the photoelectron, and a close-coupling model. In the latter two cases, equations have been derived for the optimization of the continuum orbitals required for the description of the outgoing photoelectron. Approximations which make the optimization equations more tractable are also discussed. 26 references.