Description of a two-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 radiation field using interparticle coordinates to describe the internal motion; electron correlation is fully incorporated. No restriction is placed on the total orbital angular momentum of the system. We have applied this formulation using the perimetric combination of interparticle coordinates to the calculation of accurate second- and fourth-order ac shifts and widths of the negative hydrogen ion. In addition, we have obtained estimates of the ac shifts and widths in the nonperturbative regime, using a three-level model Hamiltonian constructed from the zeroth-, first-, and second-order perturbed wave functions. We have used this model to investigate the intensity dependence of absorption profiles in the autoionizing resonance region below the two-electron escape threshold. We have also used the model to estimate the intensity for onset of stabilization (against ionization) at frequencies above the threshold for two-electron escape; the onset intensity increases as the third power of the frequency, in accord with earlier results for atomic hydrogen.