Ionization of excited hydrogen atoms by elliptically polarized fields: experiment vs. theories

We present experimental results for the ionization of hydrogen atoms with an initial principal quantum number n0 in the range 31–45, by an elliptically polarized (EP), ω/2π=9.904 GHz microwave electric field with peak amplitude F. For this range, the scaled frequency n03ω is relatively low, 0.04487–0.1372. Some data show local maxima and minima in the F-dependence of the ionization probability Pion. Classical 3d Monte Carlo calculations carried out by Richards quantitatively reproduce the striking sensitivity of this behavior to EP, and classical theory shows it to be a resonant dynamical process. A different theoretical approach by Oks and Uzer uses quantal Floquet theory to show how this driven system is related to a hydrogen atom in crossed electric and magnetic fields. We show that their quantal theory does reproduce the position of some local experimental minima in Pion, but it also predicts some minima that do not occur and fails to predict others that do occur experimentally. Finally, we present EP experimental data for Pion vs. F that exhibit significant oscillations not anticipated by any theoretical approach. We speculate on their origin.