Positron annihilation induced Auger and gamma spectroscopies of surfaces

Weiss, A. H.; Fazleev, N. G.; Nadesalingam, M.P.; Mukherjee, S.; Xie, Songhai; Zhu, Jiaxing; Davis, B.

doi:10.1016/j.radphyschem.2006.03.053

Cited by 10 publications

(11 citation statements)

References 17 publications

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“…One should expect therefore that the vertex enhancement for core electrons of many-electron atoms, including those in condensed-matter systems, should also be significant. The accurate determination of such factors is of current interest due to their vital importance in accurately interpreting, e.g., positron-induced Auger-electron spectroscopy experiments (see e.g., [26,75,76]). In light of this, we have since applied the MBT to the calculation of annihilation of positrons with the tightly bound core electrons of many-electron atoms [4], and have found that the vertex enhancement of the core electrons is indeed significant.…”

Section: Discussionmentioning

confidence: 99%

Positron scattering and annihilation in hydrogenlike ions

Green

Gribakin

2013

Phys. Rev. A

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Diagrammatic many-body theory is used to calculate the scattering phase shifts, normalized annihilation rates Z eff , and annihilation γ spectra for positron collisions with the hydrogenlike ions He + , Li 2+ , B 4+ , and F 8+. Short-range electron-positron correlations and longer-range positron-ion correlations are accounted for by evaluating nonlocal corrections to the annihilation vertex and the exact positron self-energy. The numerical calculation of the many-body theory diagrams is performed using B-spline basis sets. To elucidate the role of the positron-ion repulsion, the annihilation rate is also estimated analytically in the Coulomb-Born approximation. It is found that the energy dependence and magnitude of Z eff are governed by the Gamow factor that characterizes the suppression of the positron wave function near the ion. For all of the H-like ions, the correlation enhancement of the annihilation rate is found to be predominantly due to corrections to the annihilation vertex, while the corrections to the positron wave function play only a minor role. Results of the calculations for s-, p-, and d-wave incident positrons of energies up to the positronium-formation threshold are presented. Where comparison is possible, our values are in excellent agreement with the results obtained using other, e.g., variational, methods. The annihilation-vertex enhancement factors obtained in the present calculations are found to scale approximately as 1 + (1.6 + 0.46)/Z i , where Z i is the net charge of the ion and is the positron orbital angular momentum. Our results for positron annihilation in H-like ions provide insights into the problem of positron annihilation with core electrons in atoms and condensed matter systems, which have similar binding energies.

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Section: Discussionmentioning

confidence: 99%

Positron scattering and annihilation in hydrogenlike ions

Green

Gribakin

2013

Phys. Rev. A

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“…One way to accomplish this is to equip a conventional scanning electron microscope with a slow positron source. Other interesting applications of monoenergetic positrons beams are low-energy positron diffraction (186) and positron-annihilation-induced Auger electron spectroscopy (23), (187)(188)(189) and original papers given therein).…”

Section: Application To Polymers Monoenergetic Positron Beams Can Bementioning

confidence: 99%

Positron Annihilation Spectroscopy

Dlubek

2008

Encyclopedia of Polymer Science and Technology

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FundamentalsThe positron is the antiparticle of the electron, ie, it has the opposite charge but otherwise the same properties. In matter, a positron may be created either by the β + decay of an unstable nucleus or from a high-energy γ-photon via e + -e − pair production. If the positron encounters an electron, both particles annihilate, ie, they are transformed into photons of an energy, which is given by the famous Einstein equation, E = 2mc 2 . Here, m is the mass of the electron, c the speed of light in vacuum, and for particles at rest E = 2 × 0.511 MeV. The selection rules for the γ-photon emission are derived from the conservation laws of energy and momentum and parity considerations. Free positrons annihilate mainly into two γ-photons of energy 0.511 MeV emitted in almost collinear directions, or with much less probability into three photons of an overall energy of 1.022 MeV (16,24,25).In nonconducting materials, such as polymers, positrons can exist as free particles (e + ) or as positronium (Ps, e + e − ), which is a positron-electron bound state. Ps was experimentally discovered by DeBenedetti and Richings in 1952 (26). The "chemical" symbol Ps for positronium appears to have been introduced by McGervey and DeBenedetti in 1959 (27). Its properties and interaction with matter were reviewed in several articles (25-30). Ps is a particle of the size of a hydrogen atom, r Ps = 0.53Å, but only the mass of two electrons, and a binding energy of 6.8 eV, that is half of the ionization energy of hydrogen. The lifetime of a Ps atom depends on the relative spin orientation of positron and electron. The para-Ps (p-Ps, singlet state, spins antiparallel) decays into two photons with a mean intrinsic lifetime (self-annihilation in a vacuum) of τ pPs 0 = 0.125 ns. The ortho-Ps (o-Ps, triplet state, spins parallel) can decay only into three photons, a much slower process than two-photon emission. Therefore, o-Ps has an intrinsic lifetime of τ oPs 0 = 142 ns. The formation probabilities of the p-Ps and o-Ps states are 0.25 and 0.75 of the total Ps yield (16,25).In condensed matter studies, 22 Na is usually used as the positron source. This isotope has a half-life of 2.6 years and emits positrons with an energy distribution that has its maximum at 0.20 MeV and ranges up to 0.54 MeV (10,16). The stopping profile of positrons is a decreasing exponential function. In materials with a density of 1 g/cm 3 50%, 90%, and 99% of positrons are stopped within a thickness of 0.170, 0.60, and 1.20 mm, respectively. The energetic positrons lose their energy rapidly via ionization and excitation of electrons and finally phonons. After a few picoseconds, the positrons are in thermal equilibrium with the ambient material. In defect-free molecular crystals, positrons migrate over a mean distance of typically 100 nm before annihilating. In defected crystals and in amorphous matter, positrons may annihilate from local free volumes. The mean positron lifetimes are typically τ e+ = 200-400 ps.

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“…One should expect therefore, that the vertex enhancement for core electrons of many-electron atoms and condensed matter systems should likewise be significantly different from unity. The accurate determination of such factors is of current interest due to their vital importance in accu- rately interpreting, e.g., positron induced Auger electron spectroscopy experiments (see e.g., [24,66,67]). In light of this, we have since applied the MBT to the calculation of annihilation of positrons with the tightly-bound core electrons of many-electron atoms [4], and have found that the vertex enhancement of the core electrons is indeed significant.…”

Section: Discussionmentioning

confidence: 99%

Positron annihilation in hydrogen-like ions

Green

Gribakin

2012

J. Phys.: Conf. Ser.

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Diagrammatic many-body theory is used to calculate the scattering phase shifts, normalized annihilation rates Z eff and annihilation γ-spectra for positron collisions with the hydrogen-like ions He + , Li 2+ , B 4+ and F 8+ . Short-range electron-positron correlations and longer-range positron-ion correlations are accounted for by evaluating nonlocal corrections to the annihilation vertex and the exact positron self-energy. The numerical calculation of the many-body theory diagrams is performed using B-spline basis sets. To elucidate the rôle of the positron-ion repulsion, the annihilation rate is also estimated analytically in the Coulomb-Born approximation. It is found that the energy dependence and magnitude of Z eff is governed by the Gamow factor that characterizes the suppression of the positron wave function near the ion. For all of the H-like ions, the correlation enhancement of the annihilation rate is found to be predominantly due to corrections to the annihilation vertex, while the corrections to the positron wave function play only a minor rôle. Results of the calculations for s, p and d-wave incident positrons of energies up to the positronium formation threshold are presented. Where comparison is possible, our values are in excellent agreement with the results obtained using other, e.g., variational, methods. The annihilation vertex enhancement factors obtained in the present calculations are found to scale approximately as 1 + (1.6 + 0.46 )/Zi, where Zi is the net charge of the ion and is the positron orbital angular momentum. Our results for positron annihilation in H-like ions provide insights into the problem of positron annihilation with core electrons in atoms and condensed matter systems, which have similar binding energies.

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Positron annihilation induced Auger and gamma spectroscopies of surfaces

Cited by 10 publications

References 17 publications

Positron scattering and annihilation in hydrogenlike ions

Positron scattering and annihilation in hydrogenlike ions

Positron Annihilation Spectroscopy

Positron annihilation in hydrogen-like ions

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