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Physical Review A

Metastable-ion lifetime studies utilizing a heavy-ion storage ring: Measurements on He−

T. Andersen, L. H. Andersen, Péter Balling, Harold K. Haugen, Preben Hvelplund, W. W. Smith, Knud Taulbjerg

Published Feb 1, 1993• 77 citations
Publisher

Abstract

The lifetime of the metastable autodetaching 1s2s2p $^{4}$${\mathit{P}}_{5/2}$ level of the $^{4}\mathrm{He}^{\mathrm{\ensuremath{-}}}$ ion has been measured by a time-of-flight technique to be 350\ifmmode\pm\else\textpm\fi{}15 \ensuremath{\mu}s using a heavy-ion storage ring. Blackbody-induced photodetachment of this weakly bound level is shown to influence the decay rate by nearly 20%, an effect not discussed previously. The magnetic-field-induced mixing effects provide additional information about the lifetime of the close-lying $^{4}$${\mathit{P}}_{3/2}$ level, yielding \ensuremath{\tau}${(}^{4}$${\mathit{P}}_{3/2}$)=12\ifmmode\pm\else\textpm\fi{}2 \ensuremath{\mu}s. The observed lifetimes are consistent with previous experimental values, but the accuracy of the $^{4}$${\mathit{P}}_{5/2}$ lifetime is improved by a factor of 6, allowing a detailed test of theoretical predictions to be performed. The $^{4}$${\mathit{P}}_{5/2}$ lifetime is significantly shorter than most recent advanced theoretical results, but in agreement with a multiconfiguration Hartree-Fock approach accurately describing the initial bound $^{4}$${\mathit{P}}_{5/2}$ and the final continuum $^{2}$${\mathit{F}}_{5/2}$ states.

DOI
10.1103/physreva.47.890

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The lifetime of the metastable autodetaching 1s2s2p ^{4}$${\mathit{P}}_{5/2} level of the 4He\ensuremath−^{4}\mathrm{He}^{\mathrm{\ensuremath{-}}}4He\ensuremath− ion has been measured by a time-of-flight technique to be 350\ifmmode\pm\else\textpm\fi{}15 \ensuremath{\mu}s using a heavy-ion storage ring. Blackbody-induced photodetachment of this weakly bound level is shown to influence the decay rate by nearly 20%, an effect not discussed previously. The magnetic-field-induced mixing effects provide additional information about the lifetime of the close-lying ^{4}$${\mathit{P}}_{3/2} level, yielding \ensuremath{\tau}{(}^{4}$${\mathit{P}}_{3/2})=12\ifmmode\pm\else\textpm\fi{}2 \ensuremath{\mu}s. The observed lifetimes are consistent with previous experimental values, but the accuracy of the ^{4}$${\mathit{P}}_{5/2} lifetime is improved by a factor of 6, allowing a detailed test of theoretical predictions to be performed. The ^{4}$${\mathit{P}}_{5/2} lifetime is significantly shorter than most recent advanced theoretical results, but in agreement with a multiconfiguration Hartree-Fock approach accurately describing the initial bound ^{4}$${\mathit{P}}_{5/2} and the final continuum ^{2}$${\mathit{F}}_{5/2} states.
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 10--100-keV ${\mathrm{Ca}}^{\mathrm{\ensuremath{-}}}$ beams in a heavy-ion storage ring was measured to be \ensuremath{\sim}490 \ensuremath{\mu}s. A combination of experimental and theoretical evidence strongly suggests that the observed decay is associated with blackbody-radiation-induced photodetachment of ${\mathrm{Ca}}^{\mathrm{\ensuremath{-}}}$(4${\mathit{s}}^{2}$4p 2^{2}2P) ground-state ions at room temperature. Blackbody-radiation effects may also explain the previously reported existence of a long-lived (290\ifmmode\pm\else\textpm\fi{}100 \ensuremath{\mu}s) metastable ${\mathrm{Ca}}^{\mathrm{\ensuremath{-}}}$(4s4p2{\mathit{p}}^{2}p2 4^{4}4P) ion [D. Hanstorp et al., Phys. Rev. Lett. 63, 368 (1989)].
Long-lived metastable ${\mathrm{He}}_{2}^{\ensuremath{-}}$ negative ions have been observed for the first time. They were produced from He2+{\mathrm{He}}_{2}^{+}He2+​ by two-step electron capture in a Cs vapor target. The apparent autodetachment decay rates have been measured at several delay times after formation, and indicate the presence of more than one state, with the lifetime of the major component exceeding 100 \ensuremath{\mu}s. The most likely configurations are ${^{4}\ensuremath{\Sigma}_{g}}^{+}$ and/or $^{4}\ensuremath{\Pi}_{g}$ states bound below ${\mathrm{He}}_{2}({^{3}\ensuremath{\Sigma}_{g}}^{+})$.
We have developed a small purely electrostatic ion-beam trap which may be operated in thermal equilibrium at precisely controlled temperatures down to 10 K. Thus, we avoid magnetic field induced mixing of quantum states and may effectively eliminate any influence from absorption of photons from blackbody radiation. We report the first correction-free measurement of the lifetime of the 1s2s2p {4}P{5/2}{0} level of 4He(-) yielding the high-precision result 359.0 +/- 0.7 micros. This result is an essential proof-of-principle for cryogenic electrostatic storage rings and traps for atomic and molecular physics.
The lifetime of the 3P0^{3}P_{0}3P0​ state of ${\mathrm{Bi}}^{\ensuremath{-}}$ has been measured by selective photodetachment in a cryogenic ion-beam storage ring. By measuring the lifetime as a function of applied laser powers and extrapolating to zero laser power, a lifetime of $16.0\ifmmode\pm\else\textpm\fi{}0.5$ s is deduced for electric quadrupole decay of the excited state to the ground sate. The result provides a stringent test of recent state-of-the art theoretical calculations.