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 pola rization dependence of this process. Experiments at omega/2 pi 9.904 G Hz used two different ranges of principal quantum number no between 29 and 98. At low scaled frequencies, Omega(0) less than or similar to 0 .1, ionization data for certain narrow ranges of no exhibit striking s ensitivity to fields with elliptical polarization not too far from cir cular polarization. Classical calculations reproduce this behavior and show it to be the result of 2 omega driving terms that appear when th e Hamiltonian is transformed to a frame rotating at omega. It shows ho w higher-dimensional dynamics can influence the ionization and be used to control it when the polarization departs from linear or circular p olarization. At higher scaled frequencies, 0.6 less than or similar to Omega(0) less than or similar to 1.4, near the onset of ionization ci rcularly and elliptically polarized data show surprising similarities with linearly polarized data in a parameter regime where the ionizatio n dynamics is dominated by the influence of the pendulumlike resonance zone centered at scaled frequency n(0)(3) omega = Omega(0) = 1. The s tabilizing influence of this zone can be understood classically, but n onclassical stability associated with quantal separatrix states at its edge is a semiclassical effect.