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Physical Review A

Lifetime measurement of He− using an electrostatic ion trap

Alex Wolf, K. G. Bhushan, I. Ben-Itzhak, N. Altstein, D. Zajfman, O. Heber, Michael Rappaport

Published Jan 1, 1999• 56 citations

Abstract

The lifetimes of the metastable $1s2s2p{ }^{4}{P}_{5/2}$ level of ${\mathrm{He}}^{\ensuremath{-}},$ as well as the lifetime of the average of the ${}^{4}{P}_{3/2}$ and ${}^{4}{P}_{1/2}$ 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 ${}^{4}{P}_{5/2}$ 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 ${}^{4}{P}_{3/2}$ and ${}^{4}{P}_{1/2}$ is $8.9\ifmmode\pm\else\textpm\fi{}0.2 \ensuremath{\mu}\mathrm{s},$ which is about 20% lower than the weighted theoretical value.

DOI: 10.1103/physreva.59.267

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The lifetimes of the metastable

1s2s2p{ }^{4}{P}_{5/2}

level of

{\mathrm{He}}^{\ensuremath{-}},

as well as the lifetime of the average of the

{}^{4}{P}_{3/2}

and

{}^{4}{P}_{1/2}

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

{}^{4}{P}_{5/2}

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

{}^{4}{P}_{3/2}

and

{}^{4}{P}_{1/2}

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}

{\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.

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 radiative lifetime of the

{\mathit{F}}_{5/2}

metastable state of

{\mathrm{Xe}}^{2+}

has been measured with a different type of electrostatic ion trap. The ion trap stores ion beams of a few keV using purely electrostatic fields and provides a large field-free region where the stored ions oscillate. A photomultiplier working in the single-photon counting mode was used to observe the 380 nm photons arising from the magnetic dipole

(M1)

transition ${}^{1}{\stackrel{\ensuremath{\rightarrow}}{{S}_{0}}}^{3}{P}_{1}$ of

{\mathrm{Xe}}^{2+}.

The present measurement yields a value of $4.46\ifmmode\pm\else\textpm\fi{}0.08 \mathrm{ms},$ which is in good agreement with previous measurements, but is almost four times more accurate.

The radiative lifetime of the

2^{1}P_{1}

and

3^{1}P_{1}

state of atomic helium has been measured by the level-crossing technique. In the experiment, a beam of

2^{1}S_{0}

metastable atoms was first produced and then excited to the

2^{1}P_{1}

or

3^{1}P_{1}

state with resonance radiation. The lifetimes were measured by observing the zero-field level-crossing signals in the detected 584- or 537-\AA{} fluorescent radiation when these

^{1}P_{1}

states decayed to the

1^{1}S_{0}

ground state. The results are $\ensuremath{\tau} (2^{1}P_{1})=(5.57\ifmmode\pm\else\textpm\fi{}0.15)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ sec and $\ensuremath{\tau} (3^{1}P_{1})=(1.73\ifmmode\pm\else\textpm\fi{}0.11)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ sec. These values are in excellent agreement with theory.

The radiative lifetime of

1s2s^{3}S_{1}

{\mathrm{Li}}^{+}

ions has been measured by a comparison of the count rate of spontaneous decay photons (210 \AA{}) to a laser-induced fluorescence signal proportional to the total number of metastable ions. The ions were confined in a rf-quadrupole trap, and the fluorescence signal used was the intercombination transition

1s2p^{3}P_{1}

to $1{s}^{2}^{1}S_{0}$ at 202 \AA{}. The

^{3}P_{1}

state was populated from

^{3}S_{1}

by absorption of laser photons at 5485 \AA{}. The result is ${\ensuremath{\tau}}_{r}=58.6\ifmmode\pm\else\textpm\fi{}12.9$ sec (95% confidence level); theory predicts ${\ensuremath{\tau}}_{r}=49.1$ sec.