Resonant production of high-lying states in the microwave ionization of Na

We report microwave ionization experiments with Rydberg states of Na using several microwave frequencies near 80 GHz. We observe substantial ionization of states as low as n = 26 with microwave pulses of 170-V/cm amplitude. Unlike experiments at 38 and 17 GHz, microwave ionization is not always accompanied by the production of the extremely high-lying states just below the limit. It only occurs when the initial Rydberg state is in multiphoton resonance with the high-lying states. The resonance condition is apparent at 80 GHz because the ponderomotive energy shift of the limit, and of the high-lying states, is small compared to the microwave frequency, even at fields strong enough to produce ionization. At the lower microwave frequencies, the ponderomotive shift exceeds the microwave frequency, ensuring that the resonance condition is met and obscuring its importance. The same is true of many laser experiments.