Polarization Dependence of Microwave "ionization" of Excited Hydrogen Atoms

After a brief review of the ionization of excited hydrogen atoms by a linearly polarized field, we discuss experimental results for the polarization dependence of this process. Experiments at ω/2π = 9.904 GHz used two different ranges of principal quantum number n$\text{}_{0}$ between 29 and 98. At low scaled frequencies, Ω ≲ 0.1, ionization data for certain narrow ranges of n$\text{}_{0}$ exhibit striking sensitivity to fields with elliptical polarization not too far from circular polarization. Classical calculations reproduce this behavior and show it to be the result of 2ω driving terms that appear when the Hamiltonian is transformed to a frame rotating at ω. It shows how higher-dimensional dynamics can influence the ionization and be used to control it when the polarization departs from linear or circular polarization. At higher scaled frequencies, 0.6 ≲ Ω$\text{}_{0}$ ≲ 1.4, near the onset of ionization circularly and elliptically polarized data show surprising similarities with linearly polarized data in a parameter regime where the ionization dynamics is dominated by the influence of the pendulumlike resonance zone centered at scaled frequency n$\text{}_{0}^{3}$ω ≡ Ω$\text{}_{0}$ = 1. The stabilizing influence of this zone can be understood classically, but nonclassical stability associated with quantal separatrix states at its edge is a semiclassical effect.