Observation of resonances in the microwave-stimulated multiphoton excitation and ionization of highly excited hydrogen atoms
The multiphoton ionization of highly excited hydrogen (45 < or = n < or = 57) for frequencies 9.4 < or = v < or = 11.6 GHz is of such high order 88 < or = k/sub 0/ < or = 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 < or = k' < or = 7 between adjacent energy levels. We show that these multiphoton transition proceed at rates comparable to the overall ionization rate. These results rule out tunneling as the dominant intense-field ionization mechanism.