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arXiv:1007.4033 · Physical Review Letters

Detecting Positron-Atom Bound States through Resonant Annihilation

V. A. Dzuba, V. V. Flambaum, G. F. Gribakin

Published Nov 9, 2010• 37 citations
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Abstract

A method is proposed for detecting positron-atom bound states by observing enhanced positron annihilation due to electronic Feshbach resonances at electron-volt energies. The method is applicable to a range of open-shell transition-metal atoms which are likely to bind the positron: Fe, Co, Ni, Tc, Ru, Rh, Sn, Sb, Ta, W, Os, Ir, and Pt. Estimates of their binding energies are provided.

DOI
10.1103/physrevlett.105.203401

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A method is proposed for detecting positron-atom bound states by observing enhanced positron annihilation due to electronic Feshbach resonances at electron-volt energies. The method is applicable to a range of open-shell transition-metal atoms which are likely to bind the positron: Fe, Co, Ni, Tc, Ru, Rh, Sn, Sb, Ta, W, Os, Ir, and Pt. Estimates of their binding energies are provided.
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An overview is presented of recent studies of the interaction of low-energy positrons with atoms and molecules using a trap-based positron beam. The beam is tunable from ~100 meV to many electron volts, with a typical energy resolution of 25 meV (FWHM). A variety of scattering processes are investigated using the properties of positron orbits in a spatially varying magnetic field. Recent scattering measurements are reviewed, and new data for electronic excitation, ionization, and positronium formation in noble gases are presented. In a separate set of experiments, positron annihilation below the threshold for positronium formation is studied in molecules as a function of positron energy. The data show large Feshbach resonances in the range of energies of the molecular vibrations. Within the framework of this model, these results provide the first experimental evidence that positrons bind to hydrocarbon molecules.
Electron-positron annihilation rates calculated directly from the electron and positron densities are known to underestimate the true annihilation rate. A correction factor, known as the enhancement factor, allows for the local increase of the electron density around the positron caused by the attractive electron-positron interaction. Enhancement factors are given for positrons annihilating with the 1s1s1s electron in H, He+{\mathrm{He}}^{+}He+, He, Li2+{\mathrm{Li}}^{2+}Li2+, and Li+{\mathrm{Li}}^{+}Li+. The enhancement factor for a free positron annihilating with He+{\mathrm{He}}^{+}He+ and He is found to be close to that of ortho-positronium (i.e., Ps in its triplet state) annihilating with these atoms. The enhancement factor for Ps-He scattering is used in conjunction with the known annihilation rate for pickoff annihilation to derive a scattering length of 1.47a01.47{a}_{0}1.47a0​ for Ps-He scattering. Further, enhancement factors for e+{e}^{+}e+-Ne and e+{e}^{+}e+-Ar annihilation are used in conjunction with the pickoff annihilation rate to estimate scattering lengths of 1.46a01.46{a}_{0}1.46a0​ for Ps-Ne scattering and 1.75a01.75{a}_{0}1.75a0​ for Ps-Ar scattering.