Use of the Green's function formalism in resonant two-photon ionisation of alkali-metal atoms. I. Formal theory

The resonant two-photon ionisation of alkali-metal atoms is analysed in the dressed-atom picture using the resolvent formalism. The authors define a ladder approximation which is explained with the help of a diagrammatic representation. They show that it is then possible to perform the infinite summations related to perturbation theory which are involved in the matrix elements of the shift operator. The theoretical framework used is the Green's function formalism extended to the spin-orbit scheme adopting the quantum defect method for the atomic model. A numerical example of such a procedure is presented in the case of the caesium atom.