IONIZATION OF HYDROGEN-ATOMS BY CIRCULARLY-POLARIZED MICROWAVES

Ionization of hydrogen Rydberg atoms by circularly polarized microwaves is studied numerically within the framework of classical mechanics. Both the simplified two-dimensional model (in which the plane of polarization coincides with the orbit plane) and a fully three-dimensional system are considered. It is shown that the ionization proceeds in the diffusive manner for all microwave frequencies except the low-frequency limit. The threshold for diffusive excitation as well as the diffusion speed is strongly dependent on the initial state of the system for smooth pulse excitation. In a high-frequency limit the ionization threshold rises sharply-the atom is much more resistant to the excitation. Two distinct regimes of stabilization windows (regions where the ionization decreases with increasing field amplitude), one in the strong short-laser-pulse domain and the other in the weak microwave domain, are identified and discussed. © 1995 The American Physical Society.