Production of Extremely High-Lying States in Strong Microwave Fields

This dissertation investigates phenomena related to interactions of Rydberg atoms with strong microwave fields in the presence of small static fields on the order of 5-10 mV/cm. New aspects of microwave ionization of alkali Rydberg atoms at 17 and 38-GHz fields are presented, and the results of a novel experiment conducted at 80 GHz are discussed. Most important, the production of extremely high-lying states is observed at all microwave frequencies. About ~5-15 of the excited atoms are found in such states after the end of strong many-cycle microwave pulse in two regimes. When the laser excitation of Rydberg population occurs during the microwave pulse, the observed spectra show a regular structure of peaks spaced by a microwave photon from the ionization limit for a wide range of binding energies, both above the continuum and far below it. When the excitation happens in zero-microwave field, atoms are also found in the extremely high-lying states. In both cases, the high-lying states are observed for low Rydberg states, in the regime where the spacing between Rydberg states is much larger than the microwave frequency. Our observations at 80 GHz suggest that this phenomenon occurs due to a multiphoton excitation of atoms to the limit and is not a result of microwave ionization of atoms, but rather a distinct process that occurs along with it.