Double-frequency microwave ionization of Na

We report the ionization of Na atoms by the simultaneous application of microwave fields of two different frequencies. We conclude that the salient features of double-frequency ionization can be readily understood. Both the hydrogenlike ‖m‖=2 states and the nonhydrogenic ‖m‖=0 and 1 states ionize when the sum of the field amplitudes, the peak field, reaches the field required for ionization by a single microwave frequency, E=1/9${\mathit{n}}^4$ and E=1/3${\mathit{n}}^5$, respectively.