Polarization independence of microwave ''ionization'' thresholds of excited hydrogen atoms near the principal resonance

We study experimentally and theoretically how the polarization of a 9.904 GHz field affects threshold amplitudes for many-photon ''ionization'' of n(0) = 70,...,98 H atoms. That peak-field thresholds for circular polarization are lower than for linear polarization (with those for elliptical polarization intermediate) in the main resonance zone is a consequence of a time-scale separation in the 3D dynamics there. When an amplitude scaling relationship given by the theory is used, the experimental data confirm that, near onset, the scaled ionization thresholds are approximately independent of polarization.