The ionization of hydrogen Rydberg atoms by circularly polarized microwaves is studied quantum mechanically in a model two-dimensional atom. We apply a combination of a transformation to the coordinate frame rotating with the field, with complex rotation approach and representation of the atomic subspace in a Sturmian-type basis. The diagonalization of resulting matrices allows us to treat exactly the ionization of atoms initially prepared in highly excited Rydberg states of principal quantum number ${\mathit{n}}_0$≊60. Similarities and differences between ionization by circularly and linearly polarized microwaves are discussed with a particular emphasis on the high-frequency regime and on the localization phenomenon. The dependence of the ionization character on the initial state (circular, elliptical, or low angular momentum state) as well as on the helicity of the polarization is discussed in detail. It is shown that, in the high-frequency chaotic regime, close encounters with the nucleus do not play a major role in the ionization process. © 1996 The American Physical Society.

• Received 23 February 1996

DOI:https://doi.org/10.1103/PhysRevA.54.691