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Physical Review Letters

Identification of the Metastable Autoionizing States in Potassium with a New rf Spectroscopic Technique

G. F. Sprott, Robert Novick

Published Aug 5, 1968• 23 citations
Publisher

Abstract

A new magnetic resonance method has been developed for studying the metastable autoionizing states previously observed in the alkali elements. With this technique, two distinct but nearly degenerate metastable states were discovered in potassium: a $(3{p}^{5}4s3d)^{4}F_{\frac{9}{2}}$ state and a $(3{p}^{5}4s4p)^{4}D_{\frac{7}{2}}$ state. In addition, a value of $Q=(5.3\ifmmode\pm\else\textpm\fi{}0.8)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}26}$ ${\mathrm{cm}}^{2}$ (uncorrected for the Sternheimer effect) has been obtained for the ${\mathrm{K}}^{39}$ nucleus. The production of polarized electrons and nuclei from these states is also discussed.

DOI
10.1103/physrevlett.21.336

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A new magnetic resonance method has been developed for studying the metastable autoionizing states previously observed in the alkali elements. With this technique, two distinct but nearly degenerate metastable states were discovered in potassium: a (3p54s3d)4F92(3{p}^{5}4s3d)^{4}F_{\frac{9}{2}}(3p54s3d)4F29​​ state and a (3p54s4p)4D72(3{p}^{5}4s4p)^{4}D_{\frac{7}{2}}(3p54s4p)4D27​​ state. In addition, a value of Q=(5.3\ifmmode±\else\textpm\fi0.8)\ifmmode×\else\texttimes\fi10\ensuremath−26Q=(5.3\ifmmode\pm\else\textpm\fi{}0.8)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}26}Q=(5.3\ifmmode±\else\textpm\fi0.8)\ifmmode×\else\texttimes\fi10\ensuremath−26 cm2{\mathrm{cm}}^{2}cm2 (uncorrected for the Sternheimer effect) has been obtained for the K39{\mathrm{K}}^{39}K39 nucleus. The production of polarized electrons and nuclei from these states is also discussed.
The lowest Rb metastable autoionizing level has been identified by a rf resonance measurement of the g factors of the various hyperfine sublevels. Of the two lowest-lying metastable state assignments, 4p5(2P)5s5p(3P)4D724{p}^{5}(^{2}P)5s5p(^{3}P)^{4}D_{\frac{7}{2}}4p5(2P)5s5p(3P)4D27​​ and 4p54d(3F)5s4F924{p}^{5}4d(^{3}F)5s^{4}F_{\frac{9}{2}}4p54d(3F)5s4F29​​, only components of the 4D72^{4}D_{\frac{7}{2}}4D27​​ were observed in the metastable beam. The atomic-beam resonance method employed resembles traditional methods except that it exploits the differential metastability of the fine-structure levels instead of magnetic deflection for state selection.
Discrete atomic energy levels lying between the first and second ionization potentials which are metastable against both auto-ionization and radiative decay have been observed in the alkali elements. These levels arise from the excitation of an electron from the outermost closed shell of the atom and have characteristic lifetimes in the microsecond region. A beam of metastable alkali atoms is produced by electron bombardment of neutral ground-state atoms and is detected by collecting the charged products of the decay. Stern-Gerlach magnetic-deflection experiments have been performed to show unambiguously that the signals arise from paramagnetic atoms and not from possible stray-photon effects in the apparatus. The excitation energies, electron-bombardment production cross sections, natural lifetimes, and the tentative spectroscopic assignments of these atoms are listed below: The energy and lifetime of the metastable lithium atom are in good agreement with the available theoretical estimates. The metastable states in the heavier alkali atoms are classified in relation to the known Ib{\mathrm{I}}^{b}Ib spectral terms. Magnetic and electric fields have been found to be effective in reducing the lifetime of the metastable atoms. The Zeeman quenching is discussed in a separate paper. The Stark quenching rate has been found to be proportional to the square of the electric field and to increase rapidly with atomic number ZZZ. It is suggested that the Stark quenching results from a second-order interaction which involves the product of the electric field and the spin-orbit or other magnetic operators.
We report the first observation of hyperfine structure and isotope shift in autoionizing states. This structure was investigated in the $5{d}_{\frac{3}{2}}\mathrm{nf} J=4,5(n=36\ensuremath{-}40)$ barium Rydberg series, excited from the recently localized metastable $5{d}^{2}^{1}G_{4}$ level. An isotope-selective detection method was used in a cw laser spectroscopic experiment on a collimated atomic beam. The experimental data provide valuable additional information for the analyses of these doubly excited series.
Simple ionic systems were studied, such as metastable autoionizing states of the negative He ion, two-photon decay spectrum of metastable He ion, optical excitation with low energy ions, and lifetime measurements of singly ionized Li and metastable He ion. Simple atomic systems were also investigated. Metastable autoionizing atomic energy levels in alkali elements were included, along with lifetime measurements of Cr-53, group 2A isotopes, and alkali metal atoms using level crossing and optical double resonance spectroscopy.
A study of the physics of a nuclear magnetic resonance gyroscope is described. Experimental results in nuclear polarization and relaxation in 3He are obtained using an optical pumping apparatus and a high resolution rf spectroscopic technique. Polarization in the metastable states of the heavy noble gases (20, 21 Ne, 129, 132 Xe) was observed using a similar optical pumping apparatus. A sensitive detection method based on optical polarization of an excited state transition is developed and compared with the conventional method of monitoring optical transmission through the optical pumping cell. A new concept for the optical pumping of odd-mass-numbered heavy noble was isotopes to provide for ground-state nuclear polarization is discussed. Significant polarization in excited state neon was observed via collisional transfer from optically pumped helium in a cell filled with a mixture of helium and neon isotopes. The measured polarization was essentially independent of the isotopic composition. (Author)