Effects of Classical Resonances on the Chaotic Microwave Ionization of Highly Excited Hydrogen Atoms

Abstract

Experimental measurements of the microwave ionization of highly excited hydrogen atoms with principal quantum numbers ranging from n = 32 to 90 are well described by a classical treatment of the nonlinear electron dynamics. In particular, the measurements of the threshold field for the onset of significant ionization exhibits a curious dependence on the microwave frequency with distinct peaks at rational values of the scaled frequency, n3Ω = 1, 2/3, 1/2, 2/5, 1/3, 1/4, 1/5, which is in excellent agreement with the predictions for the onset of classical chaos in a one-dimensional model of the experiment. In the classical theory this frequency dependence of the threshold fields is due to the stabilizing effect of nonlinear resonances ("islands") in the classical phase space which is greatly enhanced when the microwave perturbation is turned on slowly (adiabatically) as in the experiments. Quantum calculations for this one-dimensional model also exhibits this stabilizing effect due to the preferential excitation of localized quasi-energy states.