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Journal of Physics B Atomic and Molecular Physics

Millimetre resonances in Na Rydberg levels detected by field ionization: quantum defects and Stark-effect studies

Claude Fabre, P. Goy, S. Haroche

Published Apr 21, 1977• 51 citations
Publisher

Abstract

Transitions between Na Rydberg levels induced by microwave radiation in the millimetre range have been observed by field-ionization detection. These transitions, whose frequencies yield the quantum defects of the levels, appear sensitive to very weak electric field perturbations as expected. Observations of field-induced forbidden transitions and of Stark shifts in fields down to 50 mV cm -1 are reported.

DOI
10.1088/0022-3700/10/6/001

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Transitions between Na Rydberg levels induced by microwave radiation in the millimetre range have been observed by field-ionization detection. These transitions, whose frequencies yield the quantum defects of the levels, appear sensitive to very weak electric field perturbations as expected. Observations of field-induced forbidden transitions and of Stark shifts in fields down to 50 mV cm -1 are reported.
An interaction of microwave radiation with high-excited (Rydberg) atoms has been suggested to be used for radiation detection and measurement of its frequency. Our objective in the present work was to investigate experimentally and theoretically the spectrum of the microwave transition 37P-37S in sodium Rydberg atoms in a weak static electric field and to perform accurate measurements of the scalar and tensor polarizabilities of the 37P level. Knowledge of polarizabilities is required to find a magnitude of energy level shifts in atom by known static electric fields. The Na S and P states possess large quantum defects (1.347 and 0.854, respectively), so that a quadratic Stark effect in weak fields is characteristic for these levels.
Millimeter resonances corresponding to single- and double-photon transitions have been observed between Rydberg levels of sodium with principal quantum number nnn ranging from 23 to 41. The levels are prepared by laser excitation; transitions are induced by microwaves produced by a backwave oscillator working around 300 GHz; detection is performed through the selective field-ionization technique. Experimental values for quantum defects, fine-structure intervals, and polarizabilities are reported.
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.
Multiphoton ionization (MPI) via two-photon resonance with high-lying Rydberg levels is observed on Na and K atoms in modest (12--120 V/cm) dc electric fields. The field-induced MPI signals are manifested to such an extent that the nominally forbidden MPI via two-photon resonance with the np levels is induced at the same signal levels as or more intensely than the allowed MPI via the ns and nd Rydberg levels. The intensity inversion between the allowed and the forbidden MPI signals is observed on the K atoms in the wavelength region where the two-photon energy is very close to the field-ionization limit.
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 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.