Circularly polarized microwave ionization of hydrogen

The low-frequency circularly polarized microwave ionization of highly excited states of the hydrogen atom is evaluated applying the adiabatic approximation in the classical limit. The ionization threshold is calculated by matching asymptotically the weak- and strong-field regimes, and the results are compared with numerical integrations of the equations of motion. Simulations for both linear and circular polarization are presented, corresponding to ensembles of initial states in experiments with linear polarized microwaves, at frequencies ω${\mathrm{\omega }}_{\mathit{K}}$, where ${\mathrm{\omega }}_{\mathit{K}}$=${\mathit{n}}^{\mathrm{-}3}$ is the initial Kepler frequency of a state with principal quantum number n. In contrast with the stochastic nature of the ionization for linearly polarized microwaves, the dynamic evolution in the case of circular polarization is quite regular and the threshold varies smoothly with frequency. As the static limit is approached, a scale-invariant switching-on procedure for the microwave field leads to a breakdown of the adiabatic approximation.