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Physical Review Letters

Excitation Spectrum of Na Rydberg States in a Strong Microwave Field: A Connection Between Two Points of View

H. B. van Linden van den Heuvell, Ravinder Kachru, Nam Huu Tran, Thomas F. Gallagher

Published Nov 12, 1984• 31 citations
Publisher

Abstract

To connect the field and photon points of view we have observed the spectra of Na atoms in the presence of weak to strong microwave fields. Using these spectra and a model based on atomic hydrogen, we connect the previous Landau-Zener description of transitions in strong, temporally varying electric fields to a description based on a photon point of view. In this regime, both descriptions predict similar microwave ionization thresholds.

DOI
10.1103/physrevlett.53.1901

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Physical Review A•Aug 1, 1991•33 citations
To connect the field and photon points of view we have observed the spectra of Na atoms in the presence of weak to strong microwave fields. Using these spectra and a model based on atomic hydrogen, we connect the previous Landau-Zener description of transitions in strong, temporally varying electric fields to a description based on a photon point of view. In this regime, both descriptions predict similar microwave ionization thresholds.
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.
Microwave ionization and excitation of Rydberg atoms provides a system in which quantitative experiments can be done to study single electron atoms in strong fields. In many cases microwave ionization proceeds by means of transitions through other bound states. Multiphoton transitions between two relatively isolated K Rydberg states have been examined under a variety of circumstances to establish the connection between transitions driven by quasi static fields and photon absorption and to understand the evolution during pulses of microwaves.
Experimental results of studying the spectrum of the microwave 37 P -37 S transition of Rydberg sodium atoms in a weak magnetic field (≤7 G) are reported. The populations of the Rydberg states were measured using the method of selective ionization with a pulsed electric field. When the magnetic field was parallel to the ionizing electric field, a good agreement between the calculated and experimental spectral shapes was observed, making it possible to determine the unknown polarization of the microwave radiation. In the case of the orthogonal configuration of the fields, the resonance structure was suppressed in the field ionization signals due to the strong influence of the magnetic field on the electron trajectories in the detection system.
The authors have observed the ionization of Na Rydberg states by strong microwave fields and have found that the field required for ionization is 1(3.7n5)\frac{1}{(3.7{n}^{5})}(3.7n5)1​ (in atomic units) for $n=20\ensuremath{-}37$. This field is close to the field 1(3n5)\frac{1}{(3{n}^{5})}(3n5)1​ at which the Stark manifolds of principal quantum numbers nnn and n+1n+1n+1 intersect. The ionization can be shown to arise from transitions occurring near these intersections.
We have studied the ionization of Na ns and nd Rydberg atoms by \ensuremath{\sim}1-\ensuremath{\mu}s pulses of 10-MHz, 670-MHz, 2-GHz, and 4-GHz fields, bridging the gap between the previous measurements of ionization by \ensuremath{\sim}1-\ensuremath{\mu}s rise-time pulses and 8- and 15-GHz microwave fields. We have also studied the ionization of Li ns and nd Rydberg atoms by 4-, 8-, and 15-GHz fields, which, due to the smaller Li quantum defects, represents the high-frequency limit of nonhydrogenic microwave ionization. The Na measurements show that ionization at 10 MHz occurs without change in n at fields of 1/16n4{\mathit{n}}^{4}n4 and 1/9n4{\mathit{n}}^{4}n4. At 670 MHz \ensuremath{\Vert}m\ensuremath{\Vert}=0, 1, and 2 states are all ionized at fields between 1/3n5{\mathit{n}}^{5}n5 and 1/9n4{\mathit{n}}^{4}n4, indicating that transitions to higher-n states are occurring. As the frequency is raised further, to 2 and 4 GHz, the \ensuremath{\Vert}m\ensuremath{\Vert}=0 and 1 states exhibit an increasingly sharp threshold for ionization at E=1/3n5{\mathit{n}}^{5}n5, but the \ensuremath{\Vert}m\ensuremath{\Vert}=2 states are unlikely to ionize at this field, with ionization occurring mostly at higher fields, 1/3n5{\mathit{n}}^{5}n5E1/9n4{\mathit{n}}^{4}n4, approaching the 15-GHz behavior of ionization at E=1/9n4{\mathit{n}}^{4}n4. At 4 GHz, Li n=20 \ensuremath{\Vert}m\ensuremath{\Vert}=0 and 1 states ionize at E=1/3n5{\mathit{n}}^{5}n5, but at n\ensuremath{\simeq}45, \ensuremath{\Vert}m\ensuremath{\Vert}=1 states ionize at E=1/9n4{\mathit{n}}^{4}n4, and the \ensuremath{\Vert}m\ensuremath{\Vert}=0 states ionize at E>1/3n5{\mathit{n}}^{5}n5. The \ensuremath{\Vert}m\ensuremath{\Vert}=2 states appear to ionize at E=1/9n4{\mathit{n}}^{4}n4. As the frequency is raised to 8 and then to 15 GHz, the transition of \ensuremath{\Vert}m\ensuremath{\Vert}=0 and 1 states to ionization at E=1/9n4{\mathit{n}}^{4}n4 occurs at lower n. When a small static field is applied, the Li ionization threshold fields drop from 1/9n4{\mathit{n}}^{4}n4 to fields somewhat above 1/3n5{\mathit{n}}^{5}n5. Together the Na and Li measurements show that microwave ionization depends on a rate-limiting step, which is a resonant multiphoton transition.