Variational methods for high-order multiphoton processes

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Abstract
Methods for applying the variationally stable procedure for Nth-order perturbative transition matrix elements of Gao and Starace [ Phys. Rev. Lett. 61, 404 ( 1988); Phys. Rev. A 39, 4550 ( 1989)] to multiphoton processes involving systems other than atomic H are presented. Three specific cases are discussed: one-electron ions or atoms in which the electron-ion interaction is described by a central potential; two-electron ions or atoms in which the electronic states are described by the adiabatic hyperspherical representation; and closed–shell ions or atoms in which the electronic states are described by the multiconfiguration Hartree–Fock representation. Applications are made to the dynamic polarizability of He and the two-photon ionization cross section of Ar.