Resonance overlap criterion for H atom ionization by circularly polarized microwave fields

The threshold for H atom ionization by circularly polarized microwave fields is discussed within the classical mechanics framework for high microwave frequencies. The Chirikov resonance overlap criterion predictions are compared with estimates obtained adopting the renormalization method. It is shown that the ionization threshold is highly sensitive to the helicity of microwaves. Among all possible initial electronic orbits, those of medium eccentricity are the first to ionize. The results obtained indicate that collisions with the nucleus play a negligible role for the onset of ionization. copyright 1997 The American Physical Society