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Physical Review A

E1−E1 damping interference in the electric field quenching of spin-polarized He+2s12 ions

Adriaan J. van Wijngaarden, Reinhard K. B. Helbing, J. Patel, G. W. F. Drake

Published Feb 1, 1982• 9 citations
Publisher

Abstract

When a beam of spin-polarized metastable ${\mathrm{He}}^{+}2{s}_{\frac{1}{2}}$ ions is quenched by an electric field $\stackrel{\ensuremath{\rightarrow}}{\mathrm{E}}$, the emitted radiation intensity contains an asymmetry term proportional to $(\stackrel{^}{k}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{^}{E})(\stackrel{\ensuremath{\rightarrow}}{\mathrm{P}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{^}{k}\ifmmode\times\else\texttimes\fi{}\stackrel{^}{E})$ where $\stackrel{\ensuremath{\rightarrow}}{\mathrm{P}}$ is the spin-polarization vector and $\stackrel{^}{k}$ is the direction of observation. The resulting asymmetry is nearly proportional to the level width of the $2{p}_{\frac{1}{2}}$ state in ${\mathrm{He}}^{+}$. The experiment provides the first observation of the asymmetry. The measured asymmetry is (0.007 69\ifmmode\pm\else\textpm\fi{}0.000 10) in agreement with the theoretical value 0.007 618 and corresponds to a lifetime ${\ensuremath{\tau}}_{2p}=(0.988\ifmmode\pm\else\textpm\fi{}0.013)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ sec.

DOI
10.1103/physreva.25.862

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When a beam of spin-polarized metastable He+2s12{\mathrm{He}}^{+}2{s}_{\frac{1}{2}}He+2s21​​ ions is quenched by an electric field E\ensuremath→\stackrel{\ensuremath{\rightarrow}}{\mathrm{E}}E\ensuremath→​, the emitted radiation intensity contains an asymmetry term proportional to (\stackrel{^}{k}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{^}{E})(\stackrel{\ensuremath{\rightarrow}}{\mathrm{P}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{^}{k}\ifmmode\times\else\texttimes\fi{}\stackrel{^}{E}) where P\ensuremath→\stackrel{\ensuremath{\rightarrow}}{\mathrm{P}}P\ensuremath→​ is the spin-polarization vector and \stackrel{^}{k} is the direction of observation. The resulting asymmetry is nearly proportional to the level width of the 2p122{p}_{\frac{1}{2}}2p21​​ state in He+{\mathrm{He}}^{+}He+. The experiment provides the first observation of the asymmetry. The measured asymmetry is (0.007 69\ifmmode\pm\else\textpm\fi{}0.000 10) in agreement with the theoretical value 0.007 618 and corresponds to a lifetime \ensuremathτ2p=(0.988\ifmmode±\else\textpm\fi0.013)\ifmmode×\else\texttimes\fi10\ensuremath−10{\ensuremath{\tau}}_{2p}=(0.988\ifmmode\pm\else\textpm\fi{}0.013)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\ensuremathτ2p​=(0.988\ifmmode±\else\textpm\fi0.013)\ifmmode×\else\texttimes\fi10\ensuremath−10 sec.
The lifetimes of the metastable 1s2s2p4P5/21s2s2p{ }^{4}{P}_{5/2}1s2s2p4P5/2​ level of ${\mathrm{He}}^{\ensuremath{-}},$ as well as the lifetime of the average of the 4P3/2{}^{4}{P}_{3/2}4P3/2​ and 4P1/2{}^{4}{P}_{1/2}4P1/2​ levels, have been measured using a new type of ion trap that stores keV ion beams using electrostatic fields only. The use of a pure electrostatic field avoids the complication of magnetic-field-induced mixing effects, which can interfere with the measurement of the spontaneous decay. The measured lifetime for the 4P5/2{}^{4}{P}_{5/2}4P5/2​ state, after correction for decay induced by blackbody radiation, is $343\ifmmode\pm\else\textpm\fi{}10 \ensuremath{\mu}\mathrm{s}.$ This value is consistent with previous experiments, and in excellent agreement with the most recent theoretical calculations. The average lifetime of the 4P3/2{}^{4}{P}_{3/2}4P3/2​ and 4P1/2{}^{4}{P}_{1/2}4P1/2​ is $8.9\ifmmode\pm\else\textpm\fi{}0.2 \ensuremath{\mu}\mathrm{s},$ which is about 20% lower than the weighted theoretical value.
The lifetime of the metastable autodetaching 1s2s2p 4^{4}4P5/2{\mathit{P}}_{5/2}P5/2​ level of the $^{4}\mathrm{He}^{\mathrm{\ensuremath{-}}}$ ion has been measured by a time-of-flight technique to be 350\ifmmode\pm\else\textpm\fi{}15 \ensuremath{\mu}s using a heavy-ion storage ring. Blackbody-induced photodetachment of this weakly bound level is shown to influence the decay rate by nearly 20%, an effect not discussed previously. The magnetic-field-induced mixing effects provide additional information about the lifetime of the close-lying 4^{4}4P3/2{\mathit{P}}_{3/2}P3/2​ level, yielding \ensuremath{\tau}(4{(}^{4}(4P3/2{\mathit{P}}_{3/2}P3/2​)=12\ifmmode\pm\else\textpm\fi{}2 \ensuremath{\mu}s. The observed lifetimes are consistent with previous experimental values, but the accuracy of the 4^{4}4P5/2{\mathit{P}}_{5/2}P5/2​ lifetime is improved by a factor of 6, allowing a detailed test of theoretical predictions to be performed. The 4^{4}4P5/2{\mathit{P}}_{5/2}P5/2​ lifetime is significantly shorter than most recent advanced theoretical results, but in agreement with a multiconfiguration Hartree-Fock approach accurately describing the initial bound 4^{4}4P5/2{\mathit{P}}_{5/2}P5/2​ and the final continuum 2^{2}2F5/2{\mathit{F}}_{5/2}F5/2​ states.
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It is shown that in the Stark quenching of a spin-polarized beam of metastable hydrogenic ions, a measurable right-left asymmetry can be produced in the emitted radiation which is proportional to the degree of spin polarization. The effect, which has previously been observed in absorption in neutral cesium and thallium, is due to interference between spontaneous magnetic dipole and induced electric dipole decay modes. Asymmetry measurements offer a simple means of determining the degree of spin polarization in the heavier hydrogenic ions.
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