Transition matrix method for multiphoton ionization processes

Abstract

A transition matrix theory is developed to treat effects of electron correlations on two-photon ionization transitions in closed shell atoms and ions in the Random Phase Approximation (RPA). The theory extends the treatment of Chang and Fano [I] for slngle-photon ionization of closed shell atoms. The electromagnetic field interaction is treated in second order perturbation theory and electron correlations of the RPA type are included to infinite order. Ground and excited intermediate states are represented by a sum of configurations having a pair of virtually excited electrons in addition to the ground state or singly excited configurations. It Is found that only one-partlcle functions, representing certain projections of excited two-partlcle wavefunctlons, need to be calculated for the intermediate state in order to describe electron correlation in the RPA. The transition matrix equations for the unknown single particle functions in the intermediate and final states are derived using the graphical method of Starace and Shahabl [2]. The summations over intermediate states, including the contlnuum, is represented by the solution of an inhomogeneous set of equations for the effective intermediate state by the wellknown Dalgarno-Lewls method [3]. Solutions of the equations allow one to obtain non-resonant two-photon ionization cross sections, photoelectron angular distributions, etc.