Ionization of Li Rydberg atoms by 8.5- and 18-GHz circularly polarized microwave fields

We have measured separately the ionization threshold fields of the sets of Li states composed of l+m even and l+m odd zero field states. In spite of the fact that the states composed of l+m even states are, by one measure, more nonhydrogenic than those composed of the l+m odd states, we observe that both sets of states ionize at a microwave field well below the hydrogenic ionization field. The ionization of the states composed of l+m odd states shows that even small departures from hydrogen lead to completely nonhydrogenic ionization, as in a static field. In contrast, in a linearly polarized microwave field small departures from hydrogen do not lead to nonhydrogenic ionization. The excitation spectrum of the states composed of l+m odd states exhibits regular structure, while that of the states composed of l+m even states does not, a difference due to the differing energy shifts of m=0 and ±1 states on transformation between the laboratory and rotating frames. © 1996 The American Physical Society.