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Physical Review A

Microwave ionization of Na atoms in weak-to-strong static electric fields

C. Y. Lee, J. M. Hettema, Thomas F. Gallagher, C. W. S. Conover

Published Dec 1, 1992• 4 citations
Publisher

Abstract

We report measurements of the microwave ionization of Rydberg atoms as a function of static electric field, for static fields from zero to the classical ionization limit. The microwave-ionization threshold field shows clear structure as a function of the static field, in some static field regimes actually increasing as the static field is increased. The dependence of the microwave field can be understood by considering the dynamics of the levels in a temporally varying field.

DOI
10.1103/physreva.46.7048

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Journal of Physics B Atomic Molecular and Optical Physics•Aug 14, 1996•2 citations
We report measurements of the microwave ionization of Rydberg atoms as a function of static electric field, for static fields from zero to the classical ionization limit. The microwave-ionization threshold field shows clear structure as a function of the static field, in some static field regimes actually increasing as the static field is increased. The dependence of the microwave field can be understood by considering the dynamics of the levels in a temporally varying field.
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< 5027<~n<~5027< n< 50 in the precessing total field Ft,{F}_{t},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,n>~45,n> 45, Ft{F}_{t}Ft​ decreases from 1/16n4{1/16n}^{4}1/16n4 to 0.8/16n4{0.8/16n}^{4}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,{F}_{t},Ft​, which we attribute to resonantly enhanced ionization, with the resonances driven by a stray static field in the x-y plane.
Electric field ionization of excited Na atoms by microwave fields between 12 and 15 GHz exhibits several interesting phenomena. The ∣m∣=0and1|m|=0 \mathrm{and} 1∣m∣=0and1 states may be ionized with a very low microwave field, $\ensuremath{\sim}\frac{1}{{3n}^{5}}$, corresponding to the point at which the nnn and n+1n+1n+1 Stark manifolds of principal quantum numbers nnn and n+1n+1n+1 intersect. The ionization occurs by an atom's making a succession of Landau-Zener transitions to higher-lying states, finally reaching the classical ionization limit. The ∣m∣=2|m|=2∣m∣=2 states, on the other hand, ionize at a very high microwave field, $\ensuremath{\sim}\frac{1}{{9n}^{4}}$. Both these observations and the more familiar static or low-frequency result of ionization by a field of $\ensuremath{\sim}\frac{1}{{16n}^{4}}$ are all consistent with ionization in a completely classical fashion. The apparent differences are due to the dynamics of passing from zero field to a high ionizing field.
We have measured the microwave ionization threshold electric fields for ns-state Na Rydberg atoms from n=24--33 as a function of the duration of the ionizing microwave pulse. Pulses as short as 2.8 ns were used, containing fewer than 25 cycles of 8.55-GHz radiation. While the fields required to produce the onset of ionization were independent of pulse duration and exhibited the ${\mathit{n}}^{\mathrm{\ensuremath{-}}5}$ scaling characteristic of ionization by diffusion through higher-energy states, the fields required to produce nearly complete ionization increased dramatically as the pulse was shortened. Additional measurements of the populations of bound states after the shortest microwave pulses showed that up to \ensuremath{\sim}10% of the atoms were excited to states above the initially populated ns state and the adjacent (n-1) manifold of higher angular momentum states, while most either ionized or remained in these states. From this observation we conclude that the rate-limiting process for ionization is the transition from the initially populated ns states or the adjacent manifold to the next higher manifold, not the diffusion through higher-lying states.
In this chapter experiments on rubidium Rydberg atoms in external fields are reviewed [1 – 8]. In the first part results on the interaction of microwave radiation with rubidium Rydberg atoms are described. The Rydberg atoms interact with a microwave pulse of a well-defined duration. When applying coherent microwave radiation with a frequency exceeding a certain value, the experimentally observed microwave field strength at which 10% of the atoms are ionized has to be much larger than the value found by classical trajectory calculations. The atoms are stabilized by the dynamical localization. An extremely weak microwave broadband noise which is added to the coherent wave at least partially destroys the localization. This phenomenon depends on the bandwidth of the added noise. In further experiments the dependence of the 10% ionization microwave field strength on the duration τ of the microwave pulse has been investigated. For increasing noise level the 10% ionization field strength ∈ 0.1 develops from a behavior ∈ 0.1 ∼ τ −1/4 to a ∈ 0.1 ∼ τ −1/2 dependence, which is characteristic for a classical diffusion process. The second part of the chapter deals with experiments in strong static crossed electric and magnetic fields. Below as well as above the ionization energy the observed quasi-Landau(QL)-resonances are explained using classical trajectories. The ionization behavior of the QL-resonances is clearly influenced by the spatial structure of the associated classical orbits. In further experiments the dependence of the observed ionization energy on the magnetic field was investigated in the presence of a fixed static electric field.
We have measured the 17.3 GHz circularly polarized microwave ionization (CPMI) threshold fields for Na Rydberg atoms and compared them to the previous 8.5 GHz measurements. In both cases the ionization fields are near 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 substantially lower fields, approaching . In addition, elliptically 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 below the CPMI threshold fields are observed in both cases at 8, but not at 17, GHz. Sequences of resonant microwave transitions are proposed to explain these resonant structures.