Positron or Positroniumlike Surface State on Al(100)?

Lynn, K. G.; Mills, A. P.; West, R N; Berko, S.; Canter, K. F.; Roellig, L. O.

doi:10.1103/physrevlett.54.1702

Cited by 74 publications

(20 citation statements)

References 11 publications

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“…Before discussing the Ps spectrum, we note a major difference between the HOPG "symmetric" component and the corresponding component in a metal 8 ; in metals the symmetric spectrum is due to positron annihilation at the surface ("e + surface state"), and its shape is substantially different from the bulk spectrum. In HOPG, the similarity with the bulk indicates that either the positron surface state does not exist and the thermalized positrons annihilate in the last few carbon layers or the electrons sampled by the surface-bound positron are very similar to those sampled by the "interlayer positrons" 7 in the bulk.…”

Section: Figures 1(a) and 1(b) Contain Typical Room-tempera-mentioning

confidence: 99%

Evidence for phonon-assisted positronium emission from graphite

et al. 1988

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We observe positronium (Ps) emission from a graphite surface being bombarded by slow positrons. The band structure of graphite does not support the usual one-electron-hole process for Ps formation because of conservation of momentum parallel to the surface. A large temperature coefficient for the emission of energetic Ps suggests a new mechanism in which momentum conservation is satisfied by the emission and absorption of phonons. A simple theory including one-phonon processes explains the temperature dependence as well as the angular distribution of the Ps.PACS numbers: 71.20. Cf, 36.10.Dr, 71.60,+z Positronium (Ps) emission from metals appears to occur primarily via the sudden capture of an electron near the surface by a thermalized positron. l The metal is left in a one-hole excited state and the Ps momentum distribution reflects reasonably well the electronic density of states at and below the Fermi surface. 2 Shakeup events involving more electron-hole pairs or phonons may be involved with lower probability, and are perhaps responsible for the observed deviation of the Ps angular distribution at low momenta from the predictions of the simplest direct capture model. 1 ' 3 In graphite, the shape of the electronic bands 4 is such that this simple Ps emission channel, although energetically allowed, is forbidden by conservation of momentum parallel to the surface. In this Letter we report that graphite does in fact emit Ps, implying the operation of a new higher-order process; a strong temperature dependence of the Ps yield suggests a phonon-assisted emission mechanism. A lowest-order theory incorporating the absorption or emission of one phonon describes well the measured momentum density of the emitted Ps as well as its temperature dependence. Slow positrons were obtained from self-moderating 64 Cu sources and were magnetically guided to an ultrahigh-vacuum (UHV) target chamber. 2 The Ps momentum densities were obtained by the two-dimensional angular correlation of annihilation radiation (2D ACAR) technique. 5 The 2D-ACAR spectra were measured with two Anger cameras, each placed 11 m from the sample. The angular resolution of the apparatus was 1 mrad full width at half maximum, with a typical coincidence rate of 25 sec ~l at the beginning of each run. The ZYAgrade 6 12x 12x1.5-mm 3 highly oriented pyrolytic graphite (HOPG) sample was cleaved in air before mounting. Prior to each several-hour run the sample was heated in situ to 1200 K under UHV conditions (10 " 10 Torr) . Figures 1(a) and 1(b) contain typical room-temperature 2D-ACAR spectra obtained at two incident positron energies: at 12 keV, where «93% of the positrons annihilate in the bulk of the sample, and at 750 eV, where they stop close to the surface. In these figures, p± and pii are the momenta perpendicular and parallel to the surface; we recall that 2D-ACAR spectra represent onedimensional integrals (along pii = p±xpn) of the 3D momentum density. In HOPG the carbon planes are aligned parallel to the surface (i.e., p± is along the c axis), wi...

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Section: Figures 1(a) and 1(b) Contain Typical Room-tempera-mentioning

confidence: 99%

Evidence for phonon-assisted positronium emission from graphite

et al. 1988

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“…I.B.I. Although significantly more involved, this technique has recently been employed by the groups at Lawrence Livermore National Laboratory and Brookhaven National Laboratory (Lynn, Mills, West, Berko, Canter, and Roellig, 1985). The 2D ACAR images are preferentially sensitive to the 2y rays from p-F$ as opposed to the 3y o-Ps annihilations, and recently they have been used for studies of the FIG.…”

Section: Positroniummentioning

confidence: 99%

“…I.E.2; Lynn, Mills, West, Berko, Canter, and Roellig, 1985) was used to measure the two-dimensional electron momentum distributions for positrons annihilating at reconstructed Si surfaces (Chen, 1987). The data (shown in Fig.…”

Section: Surface Defectsmentioning

confidence: 99%

Interaction of positron beams with surfaces, thin films, and interfaces

1988

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Recent advances in the study of solid surfaces and thin films using variable-energy positron beams are reviewed. In the first part the authors discuss the process of positron moderation and technical aspects of positron beam production and application. The second part is (roughly) organized in sections that apply to increasing time scales appropriate to the positron-solid interaction. These are (a) first encounter and scattering effects, (b) energy loss and stopping profiles,

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“…18 Solid Ne would be an easily fabricated low-Z moderator useful for producing spin-polarized slow positrons 19 and for increasing the efficiency of an intense 64 Cu slow-positron beam. 20 The high reemission coefficient for positrons with energy below the inelastic threshold could be useful in ultrahighefficiency multiple-bounce moderator geometries and in experiments requiring positron bottling and bunching techniques. 21 We would like to thank Dr. Kelvin Lynn for his helpful comments on this manuscript, and Dr. Don Eigler for useful discussions.…”

Section: Armentioning

confidence: 99%

Positron Dynamics in Rare-Gas Solids

1986

Phys. Rev. Lett.

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Using a beam of slow positrons in ultrahigh vacuum, we estimate the positron mean free path and energy-loss rate and measure the inelastic thresholds due to exciton, electron-hole pair, and positronium formation in solid rare-gas targets. The measurements are used to explain the large emission energies of positrons from the wide-band-gap solids in terms of a hot-positron model. The large diffusion length of the hot positrons in the solid rare gases makes them very efficient moderators for producing slow positrons. PACS numbers: 71.60. + Z, 78.70.Bj, 79.20.Hx There have been many studies of positrons in ionic crystals. 1 Measurements have revealed the positronium binding energies, wave function, effective mass, and formation mechanisms. Powdered insulators provided the first sources of positronium in vacuum 2,3 and the first practical efficient moderator for producing slow positrons. 4 Recently there have been studies of the positronium formation mechanism at the surface of ice, 5 of positronium emission from quartz, 6 and of positron reemission by various ionic solids. 7 "" 9 Unfortunately, this large body of work has not led to a precise picture of how a low-energy positron interacts with an insulator. The present study attempts to rectify this situation by careful measurements on the ideal wide-band-gap insulators, the rare-gas solids.When positrons are implanted into an ionic solid at kiloelectronvolt energies, one observes the reemission of positrons with a spread of kinetic energies comparable to the band-gap energy. In Ref. 9 this reemission was interpreted in terms of a modified version of the positronium breakup mechanism originally proposed by Canter et ai 4 In this model the reemitted positrons result from positronium formed in the bulk which diffuses to the surface and breaks up, emitting the positron via an Auger-iike process. We now find that positrons are copiously emitted with several electronvolts of kinetic energy from solid Ne, Ar, Kr, and Xe. However, because of the simplicity of these materials we have also been able to examine the emission process in detail by measuring the mean free path and inelastic thresholds for few-electronvolt positrons. The measurements are not consistent with positronium breakup and suggest that the emitted positrons are simply those that have not thermalized and happen to scatter back to the surface. Our new results also show that the rare-gas solids are a new class of very efficient slow-positron moderators.We propose to explain the observed positron reemission as follows. In any material, energetic positrons lose energy rapidly by making inelastic collisions involving electronic transitions. However, in an insulator there can be no more of these events once a positron has less energy than is needed to make an electron-hole pair, an exciton, or positronium. The positron continues to lose energy by creating phonons, but since the maximum phonon energy (E m&x ) is small, the diffusion length of the hot positron is large. Positrons reaching the surface before their...

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Positron or Positroniumlike Surface State on Al(100)?

Cited by 74 publications

References 11 publications

Evidence for phonon-assisted positronium emission from graphite

Evidence for phonon-assisted positronium emission from graphite

Interaction of positron beams with surfaces, thin films, and interfaces

Positron Dynamics in Rare-Gas Solids

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