CIRCULARLY AND ELLIPTICALLY POLARIZED MICROWAVE IONIZATION OF NA RYDBERG ATOMS

We have measured the 17.3 GHz circularly polarized microwave ionizatio n (CPMI) threshold fields for Na Rydberg atoms and compared them to th e previous 8.5 GHz measurements. In both cases the ionization fields a re near E = 1/16n(4) for low n and fall below this value at high n. A small ellipticity in the polarization and a small static field in the plane of the rotating microwave field both lead to ionization at subst antially lower fields, approaching E = 1/3n(5). In addition, elliptica lly polarized microwave ionization (EPMI) of Rydberg atoms and CPMI in the presence of a small static field have much lower threshold fields for ionization and unexpected resonant peaks at microwave fields belo w the CPMI threshold fields are observed in both cases at 8, but not a t 17, GHz. Sequences of resonant microwave transitions are proposed to explain these resonant structures.