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Physical Review Letters

Observation of Cold, Long-Lived Antiprotonic Helium Ions

M. Hori, J. A. Eades, R. Hayano, W. Pirkl, E. Widmann, H. Yamaguchi, H. A. Torii, Bertalan Juhász, D. Horváth, Ken Suzuki, T. Yamazaki

Published Feb 14, 2005• 45 citations
Publisher

Abstract

Cold, two-body antiprotonic helium ions $\overline{p}\text{ }^{4}\mathrm{H}\mathrm{e}^{2+}$ and $\overline{p}\text{ }^{3}\mathrm{H}\mathrm{e}^{2+}$ with 100-ns-scale lifetimes, occupying circular states with the quantum numbers ${n}_{i}=28--32$ and ${\ensuremath{\ell}}_{i}={n}_{i}\ensuremath{-}1$ have been observed. They were produced by cooling three-body antiprotonic helium atoms in an ultra-low-density helium target at temperature $T\ensuremath{\sim}10\text{ }\text{ }\mathrm{K}$ by atomic collisions, and then removing their electrons by inducing a laser transition to an autoionizing state. The lifetimes of $\overline{p}\text{ }^{3}\mathrm{H}\mathrm{e}^{2+}$ against annihilation induced by collisions were shorter than those of $\overline{p}\text{ }^{4}\mathrm{H}\mathrm{e}^{2+}$, and decreased for larger-${n}_{i}$ states.

DOI
10.1103/physrevlett.94.063401

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Cold, two-body antiprotonic helium ions p‾ 4He2+\overline{p}\text{ }^{4}\mathrm{H}\mathrm{e}^{2+}p​ 4He2+ and p‾ 3He2+\overline{p}\text{ }^{3}\mathrm{H}\mathrm{e}^{2+}p​ 3He2+ with 100-ns-scale lifetimes, occupying circular states with the quantum numbers ni=28−−32{n}_{i}=28--32ni​=28−−32 and \ensuremathℓi=ni\ensuremath−1{\ensuremath{\ell}}_{i}={n}_{i}\ensuremath{-}1\ensuremathℓi​=ni​\ensuremath−1 have been observed. They were produced by cooling three-body antiprotonic helium atoms in an ultra-low-density helium target at temperature T\ensuremath∼10  KT\ensuremath{\sim}10\text{ }\text{ }\mathrm{K}T\ensuremath∼10  K by atomic collisions, and then removing their electrons by inducing a laser transition to an autoionizing state. The lifetimes of p‾ 3He2+\overline{p}\text{ }^{3}\mathrm{H}\mathrm{e}^{2+}p​ 3He2+ against annihilation induced by collisions were shorter than those of p‾ 4He2+\overline{p}\text{ }^{4}\mathrm{H}\mathrm{e}^{2+}p​ 4He2+, and decreased for larger-ni{n}_{i}ni​ states.
Calculations of low-energy collisions of antiprotons p‾\overline{p}p​ with helium atoms in the ground (11S)(1\phantom{\rule{4pt}{0ex}}^{1}\mathit{S})(11S) and metastable (23S(2\phantom{\rule{4pt}{0ex}}^{3}\mathit{S}(23S and 21S)2\phantom{\rule{4pt}{0ex}}^{1}\mathit{S})21S) states are carried out by using a semiclassical method and by further assuming that one of the two electrons is passive. Antiproton capture to form antiprotonic helium $(\ensuremath{\rightarrow}\overline{p}{\mathrm{He}}^{+}+e)$ and breakup ionization $(\ensuremath{\rightarrow}\overline{p}+{\mathrm{He}}^{+}+e)$ are investigated. The energy dependence of the capture and ionization cross sections and the distributions of the product states are presented and compared with previous theoretical calculations. For the metastable helium targets, most of the p‾He+\overline{p}{\mathrm{He}}^{+}p​He+ atoms produced in the capture have long lifetimes against subsequent Auger decays and can be observed experimentally. For the ground-state targets, however, the formation of such long-lived states is rather a rare event at low collision energies.
Antiproton capture by lithium atoms ($\overline{p}+\mathrm{Li}\ensuremath{\rightarrow}\overline{p}{\mathrm{Li}}^{+}+e$) is investigated at collision energies from 0.010.010.01 to 10 eV by using a semiclassical (also know as quantum-classical hybrid) method, in which the radial distance between the antiproton and the Li+{}^{+}+ ion is treated as a classical variable, and the other degrees of freedom are described by quantum mechanics. Analyzing the wave packet of the emitted electrons and making use of the energy conservation rule enable us to calculate the state distribution of the produced antiprotonic lithium p‾Li+\overline{p}{\mathrm{Li}}^{+}p​Li+ atoms and also to distinguish between the capture and ionization ($\ensuremath{\rightarrow}\overline{p}+{\mathrm{Li}}^{+}+e$) channels at collisional energies above the ionization threshold. This method is tested for the capture of negative muons by hydrogen atoms, which was rigorously investigated in previous quantum mechanical studies. Most of the p‾Li+\overline{p}{\mathrm{Li}}^{+}p​Li+ atoms produced in p‾+Li\overline{p}+\mathrm{Li}p​+Li are found to be sufficiently stable against Auger decays and are experimentally observable as long-lived states. The present system bears close similarities to the system of p‾+He(2S)\overline{p}+\mathrm{He}(2S)p​+He(2S). It is therefore expected that long-lived antiprotonic helium p‾He+\overline{p}{\mathrm{He}}^{+}p​He+ atoms can be efficiently produced in the p‾\overline{p}p​ capture by metastable He(23S2\phantom{\rule{0.16em}{0ex}}{}^{3}S23S) atoms.
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Laser studies of metastable antiprotonic helium atoms, which we recently initiated by observing a sharp increase of the antiproton annihilation rate induced by laser-stimulated resonant transitions, have been extended. With a single laser tuned to the resonance already found at 597.26 nm, we have now established the time dependence of the upper state population. With two lasers ignited at variable time separation, we also studied the feeding of the upper state from higher atomic levels. The initial populations and level lifetimes of excited exotic atoms were determined.
A systematic study of the decay rates of antiprotonic helium (p‾4He+{\overline{p}}^{4}{\mathrm{He}}^{+}p​4He+ and p‾3He+{\overline{p}}^{3}{\mathrm{He}}^{+}p​3He+) at CERN Antiproton Decelerator has been made by a laser spectroscopic method. The decay rates of some of its short-lived states, namely those for which the Auger rates ${\ensuremath{\gamma}}_{A}$ are much larger than their radiative decay rates $({\ensuremath{\gamma}}_{\mathrm{rad}}\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}{s}^{\ensuremath{-}1})$, were determined from the time distributions of the antiproton annihilation signals induced by laser beams, and the widths of the atomic resonance lines. The magnitude of the decay rates, especially their relation with the transition multipolarity, is discussed and compared with theoretical calculations.
The exotic helium atom, consisting of a helium nucleus He2+{\mathrm{He}}^{2+}He2+, an electron ${\mathit{e}}^{\mathrm{\ensuremath{-}}}$, and a massive, negatively charged particle ${\mathit{X}}^{\mathrm{\ensuremath{-}}}$, such as an antiproton (p\ifmmode\bar\else\textasciimacron\fi{}) or a kaon (${\mathit{K}}^{\mathrm{\ensuremath{-}}}$), is studied theoretically. This exotic atom is more like a polar molecule with two nuclei He2+{\mathrm{He}}^{2+}He2+ and ${\mathit{X}}^{\mathrm{\ensuremath{-}}}$, if ${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ is in a low-lying orbital and ${\mathit{X}}^{\mathrm{\ensuremath{-}}}$ is in a highly excited orbital. Such states are generally formed in the capture of ${\mathit{X}}^{\mathrm{\ensuremath{-}}}$ by helium. The Born-Oppenheimer (BO) separation of the ${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ motion and the He2+{\mathrm{He}}^{2+}He2+-${\mathit{X}}^{\mathrm{\ensuremath{-}}}$ motion is a good approximation for these states, and affords a transparent, unified perspective of exotic helium with different ${\mathit{X}}^{\mathrm{\ensuremath{-}}}$ and different isotopes of He2+{\mathrm{He}}^{2+}He2+. A propensity rule that favors small transition energies (except for the smallest one) is found for radiative emission by low vibrational and high rotational levels. This rule resembles the selection rule for infrared emission by the usual diatomic polar molecules, and contradicts the conventional expectation that radiative transitions in exotic atoms favor larger transition energies. The radiative lifetimes \ensuremath{\tau} of highly excited states formed immediately after the capture of ${\mathit{X}}^{\mathrm{\ensuremath{-}}}$ are of the order of \ensuremath{\mu}sec for 4He2+^{4}\mathrm{He}^{2+}4He2+-p\ifmmode\bar\else\textasciimacron\fi{}-${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ and 3He2+^{3}\mathrm{He}^{2+}3He2+-p\ifmmode\bar\else\textasciimacron\fi{}-${\mathit{e}}^{\mathrm{\ensuremath{-}}}$, and those for 4He2+^{4}\mathrm{He}^{2+}4He2+-${\mathit{K}}^{\mathrm{\ensuremath{-}}}$-${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ are about half the values for the antiprotonic helium.The effect of the electronic charge polarization by ${\mathit{X}}^{\mathrm{\ensuremath{-}}}$ on \ensuremath{\tau} is explained quantitatively in a simple picture in terms of the molecular dipole moment. Low vibrational, high rotational states are described as circular or near-circular states in the atomic picture. They are stable against Auger decay, which the nonadiabatic perturbation on the BO states induces. An infinite series of bound \ensuremath{\Sigma} electronic states of exotic helium negative ions (He2+{\mathrm{He}}^{2+}He2+-${\mathit{X}}^{\mathrm{\ensuremath{-}}}$-${\mathit{e}}^{\mathrm{\ensuremath{-}}}$)-${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ exists for any He2+{\mathrm{He}}^{2+}He2+-${\mathit{X}}^{\mathrm{\ensuremath{-}}}$ distances R beyond 0.819 a.u. in the BO approximation. An infinite \ensuremath{\Pi} series exists for R>3.811 a.u., and an infinite \ensuremath{\Delta} series for R>9.532 a.u.