Stochastic ionization of hydrogen atoms in a circularly polarized microwave field

Ionization of hydrogen atoms by circularly polarized microwave radiation is investigated by means of a classical Hamiltonian model exhibiting chaotic dynamics. An analytic expression is derived for the ionization threshold as a function of the microwave frequency and field strength and is found to agree well with computed Poincar\'e sections in the high-scaled-frequency regime, n₀³{ω}>1. The results underline the crucial role of core effects in the ionization of nonhydrogenic atoms. For nearly circular orbits forbidden zones of complex eccentricity partition the phase-space torus into disjoint regions.