Circularly and elliptically polarized microwave ionization of sodium Rydberg atoms

17.3 GHz circularly polarized microwave ionization (CPMI) thresholds of Na Rydberg atoms have been measured and compared to the previous 8.5 GHz measurements. The experimental approach to producing the circularly polarized microwave field is to use a Fabry Perot cavity. An angular momentum barrier model is applied to understand the threshold measurements of CPMI. The maximum amount of angular momentum that can be transferred in a circularly polarized microwave field is deduced from the experimental data.; Elliptically polarized microwave ionization (EPMI) of Rydberg atoms is studied with microwave frequencies of 8 GHz and 18 GHz. Unexpected resonant peaks below thresholds are observed at certain microwave field amplitudes. A sequence of resonant microwave transitions is proposed to explain the resonant structures in EPMI. The relation between the dressed state map and the structures of EPMI scans is verified.