A semiclassical ionization mechanism for excited hydrogen in high-frequency fields

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Abstract
The authors introduce a new way of approximating transitions into the continuum for those circumstances where the bound and continuum states are coupled only through the high-lying levels. The method uses classical perturbation theory to approximate the dynamics over short times: it produces transition amplitudes agreeing with Fermi's golden rule in the limit of weak fields and one-photon transitions and classical dynamics in the opposite extreme where many photons are needed to reach the continuum. They apply the theory to earlier experiments measuring ionization of a highly excited hydrogen atom by the simultaneous action of microwave and static electric fields, and show that their values for the field producing 10% ionization agree reasonably well with those of the experiment.