DESCRIPTION OF A 2-ELECTRON ATOM OR ION IN AN AC FIELD USING INTERPARTICLE COORDINATES, WITH AN APPLICATION TO H-

We provide a general formulation of the energy eigenvalue problem for a two-electron atomic system interacting with a monochromatic radiatio n field using interparticle coordinates to describe the internal motio n; electron correlation is fully incorporated. No restriction is place d on the total orbital angular momentum of the system. We have applied this formulation using the perimetric combination of interparticle co ordinates to the calculation of accurate second- and fourth-order ac s hifts and widths of the negative hydrogen ion. In addition, we have ob tained estimates of the ac shifts and widths in the nonperturbative re gime, using a three-level model Hamiltonian constructed from the zerot h-, first-, and second-order perturbed wave functions. We have used th is model to investigate the intensity dependence of absorption profile s in the autoionizing resonance region below the two-electron escape t hreshold. We have also used the model to estimate the intensity for on set of stabilization (against ionization) at frequencies above the thr eshold for two-electron escape; the onset intensity increases as the t hird power of the frequency, in accord with earlier results for atomic hydrogen. [S1050-2947(97)09311-6].