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 nonre lativistic one-electron atom. The radiation field is switched on adiab atically, while the atom is assumed to be initially in its ground stat e. We analyze their interaction to the first order of perturbation the ory, taking into account retardation effects. In the radiation gauge, the exact first-order correction to the wave function consists of sepa rate orbital and spin contributions which are determined, respectively , by a vector and a scalar linear-response function. Starting from Hos tler's integral representation of the Schrodinger Coulomb Green functi on in coordinate space, we have derived them, via a generating functio n, as closed-form contour integrals. Then they have also been written explicitly, as double power series involving linear combinations of Hu mbert hypergeometric functions Phi(1). From the integral representatio n of the linear-response wave function we have extracted the considera bly simpler second-order retardation approach. We have subsequently tr anslated it in a conveniently modified Poincare-gauge, which we call a multipolar gauge, in order to display the contributions of the genuin e field-atom multipole couplings. The relevant orbital and spin multip ole terms are then recovered by employing directly the generating func tion of the linear response. Their low-and high-frequency behavior is finally examined. [S1050-2947(97)03711-6].