Observation of Resonances in the Microwave-Stimulated Multiphoton Excitation and Ionization of Highly Excited Hydrogen Atoms

Abstract

The multiphoton ionization of highly excited hydrogen ($45<~n<~57$) for frequencies $9.4<~\nu <~11.6$ GHz is of such high order $88<~k_0<~173$ that quantal tunneling may compete. Nevertheless, we observe resonances in the frequency dependence of the ionization rate caused by strong multiphoton transitions of order $3<~k^{\prime }<~7$ between adjacent energy levels. We show that these multiphoton transitions proceed at rates comparable to the overall ionization rate. These results rule out tunneling as the dominant intense-field ionization mechanism.