Two-electron atoms in superintense radiation fields: Dichotomy and stabilization

The high-frequency Floquet theory describing the interaction of a two-electron atom with a linearly polarized laser field is applied to the case when the characteristic parameter a₀=E₀w^-2 a.u. is large, corresponding to the dichotomy regime of the one-electron problem. We first revisit this case and extend the large- a₀ energy-level formula obtained earlier to higher order in a₀^-1. We then prove the existence of a dichotomy regime also for the two-electron atom, characterized by the two electrons being situated in disjoint electronic clouds separated by an average distance of 2a₀. We obtain the first four terms in the expansion of the related energy-level formula in fractional powers of a₀^-1. The coefficients entering this expansion have been expressed in terms of the eigenvalues of a nonseparable Schrödinger equation containing the end-point potential and of integrals over its eigenfunctions. The equation was solved using the finite element method. An infinite sequence of levels emerges. In the case of H^- this implies the existence of a large number of light-induced excited states, some of them corresponding to two-electron excitations not subject to autodetachment. Finally, we prove that in the dichotomy regime a two-electron atom undergoes stabilization and that the ionization rates are essentially twice those for a one-electron atom with the same nuclear charge.