  The dynamics of Rydberg states of atomic hydrogen illuminated by resonant
elliptically polarized microwaves is investigated both semiclassically and
quantum mechanically in a simplified two-dimensional model of an atom.
Semiclassical predictions for quasienergies of the system are found to be in a
very good agreement with exact quantum data enabling a classification of
possible types of motion and their dynamics with the change of the ellipticity
of the microwaves. Particular attention is paid to the dynamics of the
nonspreading wave packet states which are found to exist for an arbitrary
microwave polarization.
