In circularly polarized microwave fields of frequencies in the vicinity of 10 GHz ionization of Na Rydberg atoms occurs at a field E=1/16n4, up to principal quantum number n=50. This field is equal to the classical field for ionization and the static field required to ionize a Na atom. On the other hand, this field is far below the linearly polarized 10 GHz field required to ionize Na atoms, E=1/3n5. If the problem is transformed to a frame rotating with the microwave field, the field becomes a static field. In this case it is straightforward to calculate the ionization field classically. However, it is far lower than the experimentally observed field, a discrepancy which may be due to an angular momentum barrier.
We report the first observation of the ionization of Rydberg atoms by a circularly polarized 8.5-GHz microwave field. The observed threshold field for ionization is E=1/16
n4, a value higher than that required for ionization by a linearly polarized microwave field. The origin of this threshold can be understood by transforming to a frame rotating at the microwave frequency. In the rotating frame no transitions occur, and level mixings alone are responsible for the ionization.
We have studied in detail the continuous evolution from static field ionization to ionization by a circularly polarized microwave field by combining a z-polarized static field with a circularly polarized microwave field in the x-y plane. We have measured the ionization threshold for Na states of
27< n< 50 in the precessing total field
Ft, as a function of its angle \ensuremath{\theta} from the z axis. For low n, ${F}_{t}\ensuremath{\cong}{1/16n}^{4},$ independent of \ensuremath{\theta}. For
n> 45, Ft decreases from
1/16n4 to
0.8/16n4 as \ensuremath{\theta} is increased from 0\ifmmode^\circ\else\textdegree\fi{} to 25\ifmmode^\circ\else\textdegree\fi{}, and it remains at this value until $\ensuremath{\theta}=90\ifmmode^\circ\else\textdegree\fi{}.$ We also observe ionization at quite low values of
Ft, which we attribute to resonantly enhanced ionization, with the resonances driven by a stray static field in the x-y plane.
A classical study of the dynamics of atomic Rydberg states interacting with both elliptically and circularly polarized microwave fields is presented. In recent experiments, enhanced ionization rates were observed in elliptically as opposed to circularly polarized fields. A classical phase-space simulation provides good qualitative agreement with experimental findings. These results are explained in terms of the breakdown of almost conserved quantities as the polarization of the field is changed from circular to elliptical.
We have measured the ionization fields for Na Rydberg states of $n\ensuremath{\ge}26$ by a 79-GHz microwave field. At low
n, where the microwave frequency $\ensuremath{\omega}$ is one-third the classical Kepler frequency ${\ensuremath{\omega}}_{K}$, which is equal to the
1/n3 spacing between
n states, the ionization field is $\ensuremath{\approx}1/3{n}^{5}$, in atomic units. As
n is raised ${\ensuremath{\omega}}_{K}$ decreases, and the ionization field rises above
1/3n5, reaching a constant value when $\ensuremath{\omega}={\ensuremath{\omega}}_{K}$. At very high
n, where $\ensuremath{\omega}>1/2{n}^{2}$, single photoionization is possible, and the measured rate agrees with the calculated value. These measurements, together with those done previously, connect the processes of field ionization and photoionization in Na. At low
n and radio frequencies ionization is simply field ionization, and at very high
n it occurs by photoionization, single-photon absorption. These measurements also show that microwave ionization of Rydberg atoms at 79 GHz results in significant populations' being left in the very high-lying states.
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 \ifmmode\pm\else\textpm\fi{}1 states on transformation between the laboratory and rotating frames. \textcopyright{} 1996 The American Physical Society.