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arXiv:nucl-th/9304012 · Physical Review A

Semiclassical description of antiproton capture on atomic helium

William A Beck, L. Wilets, Mary Alberg

Published Oct 1, 1993• 49 citations
arXivHTMLPDF

Abstract

A semiclassical, many-body atomic model incorporating a momentum-dependent Heisenberg core to stabilize atomic electrons is used to study antiproton capture on helium. Details of the antiproton collisions leading to eventual capture are presented, including the energy and angular-momentum states of incident antiprotons which result in capture via single- or double-electron ionization, i.e., into ${\mathrm{He}}^{2+}$p\ifmmode\bar\else\textasciimacron\fi{} or ${\mathrm{He}}^{+}$p\ifmmode\bar\else\textasciimacron\fi{}, and the distribution of energy and angular-momentum states following the Auger cascade. These final states are discussed in light of recently reported anomalously long-lived antiproton states observed in liquid He [Iwasaki et al., Phys. Rev. Lett. 67, 1246 (1991)].

DOI
10.1103/physreva.48.2779

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AIP conference proceedings•Jan 1, 2000•2 citations
A semiclassical, many-body atomic model incorporating a momentum-dependent Heisenberg core to stabilize atomic electrons is used to study antiproton capture on helium. Details of the antiproton collisions leading to eventual capture are presented, including the energy and angular-momentum states of incident antiprotons which result in capture via single- or double-electron ionization, i.e., into He2+{\mathrm{He}}^{2+}He2+p\ifmmode\bar\else\textasciimacron\fi{} or He+{\mathrm{He}}^{+}He+p\ifmmode\bar\else\textasciimacron\fi{}, and the distribution of energy and angular-momentum states following the Auger cascade. These final states are discussed in light of recently reported anomalously long-lived antiproton states observed in liquid He [Iwasaki et al., Phys. Rev. Lett. 67, 1246 (1991)].
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.
The capture of slow antiprotons (energies less than 1.0 au, i.e. roughly 27 eV) by the rare gas atoms helium, neon and argon is considered. Appropriate to this low velocity, the capture cross sections are calculated using the adiabatic `hidden-crossing' theory in which the collision complex is viewed as a transient diatomic molecule with the positively-charged atomic ion and antiproton as nuclei. In addition to the total capture cross section, estimates of the percentage population of long-lived `circular' states is given. Our calculations suggest that a few per cent of captured antiprotons occupy these states in helium but in the case of argon or neon the probability of primary capture into such long-lived states is negligible. These results are at variance with previous calculations of antiproton capture cross sections.
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 ${\ensuremath{\ell}}_{i}={n}_{i}\ensuremath{-}1$ 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 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.
Antiprotonic atoms form in low-energy collisions between an antiproton and a normal atom or molecule. The fermion molecular dynamics (FMD) method enables us to go beyond the atomic hydrogen target and study multielectron effects with helium and neon targets and molecular effects with hydrogen and deuterium targets. Multiple ionization and ro-vibrational excitation are found to greatly increase the maximum energy of capture, with the latter leading to a significant dependence on the projectile mass and target isotope. The kinetic energies of the ionized electrons are found to increase somewhat with the target nuclear charge.