Hydrogen in intense laser fields: Radiative close-coupling equations and quantum-defect parametrization

A system of radiative close-coupling equations for a hydrogen atom in a circularly polarized intense laser field is derived. The radiative scattering matrix is parametrized within a multichannel quantum-defect formalism. The quasienergy spectrum corresponding to nonperturbative shifts and ionization widths of the bound atomic states is computed from the poles of the radiative scattering matrix. For an intensity range up to ${\mathrm{\alpha }}_0$≊1.5${\mathit{a}}_0$ (with ${\mathrm{\alpha }}_0$ the oscillation amplitude of the free electron in the laser and ${\mathit{a}}_0$ the Bohr radius), numerical results are presented in the frequency regime where two-photon ionization and above-threshold ionization of the ground state is possible. For one-photon transitions a stabilization of the atomic states for strong fields is predicted.