Linear response of a nonrelativistic hydrogenlike atom to a single-mode radiation field .1. Exact theory: The atomic ground state

In the framework of semiclassical theory we investigate the influence of a low-intensity monochromatic electromagnetic plane wave on a nonrelativistic one-electron atom. The radiation field is switched on adiabatically, while the atom is assumed to be initially in its ground state. We analyze their interaction to the first order of perturbation theory, taking into account retardation effects. In the radiation gauge, the exact first-order correction to the wave function consists of separate orbital and spin contributions which are determined, respectively, by a vector and a scalar linear-response function. Starting from Hostler's integral representation of the Schrodinger Coulomb Green function in coordinate space, we have derived them, via a generating function, as closed-form contour integrals. Then they have also been written explicitly, as double power series involving linear combinations of Humbert hypergeometric functions Phi(1). From the integral representation of the linear-response wave function we have extracted the considerably simpler second-order retardation approach. We have subsequently translated it in a conveniently modified Poincare-gauge, which we call a multipolar gauge, in order to display the contributions of the genuine field-atom multipole couplings. The relevant orbital and spin multipole terms are then recovered by employing directly the generating function of the linear response. Their low-and high-frequency behavior is finally examined. [S1050-2947(97)03711-6].