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Journal of Non-crystalline Solids | 1994

Microvoids in chalcogenide glasses studied by positron annihilation

Kjeld O. Jensen; Philip S. Salmon; Ian T. Penfold; P. G. Coleman

Abstract Microvoids have been detected in glassy As2Se3 by positron lifetime spectroscopy. The positron lifetime in perfect and defected crystalline As2Se3 was calculated and the results correlated with the size of the (open volume) defects. This correlation suggests that the observed mean lifetime τ = 345(2) ps for the glassy material corresponds to microvoids of on average about three missing atoms (i.e., 2.5–3 A radius). The lower limit of the microvoid concentration per atom is estimated to lie between 0.1 and 100 ppm (corresponding to a volume fraction 10−5−10−2%), well below the detection threshold of small-angle X-ray scattering. The microvoids are stable on heating to 260°C, i.e., after annealing at temperatures 80°C above the glass transition temperature. The present results are in conflict with earlier suggestions that positrons are trapped only in the vicinity of point-like negatively charged defects in amorphous As2Se3. Frequent observation of long positron lifetimes leads to the conclusion that microvoids are a feature common to a wide range of chalcogenide glasses. The consequence of these results for the recent model of Elliott, in which the first sharp diffraction peak in the total structure factor of covalently bonded glasses is attributed to the chemical ordering of microscopic voids around cation-centred units, is discussed.


Applied Physics Letters | 1991

Material dependence of positron implantation depths

J. A. Baker; N. B. Chilton; Kjeld O. Jensen; A. B. Walker; P. G. Coleman

New experiments and Monte Carlo simulations of positron implantation in gold are presented which, when compared with earlier work on aluminum, clearly demonstrate that the material dependence of positron implantation profiles is not adequately described by the simple mass density scaling factor in the widely used expression for median implantation depths. There is excellent agreement between the experimental results and the simulations which use the Penn dielectric formalism to describe inelastic scattering.


4th International workshop on: Slow‐positron beam techniques for solids and surfaces | 2008

Monte Carlo simulation of positron slowing down in aluminium

Kjeld O. Jensen; Alison B. Walker; Nadir Bouarissa

We have performed Monte Carlo simulations of positron slowing down in Al for incident positron energies in the range 1–10 keV. We present results for the backscattering probabilities and implantation profiles. The backscattering probabilities are significantly higher than the experimental values obtained at University of East Anglia (UEA) by Baker et al. However, the results for implantation profiles show good agreement with experimental results by the UEA group, and they indicate that the Makhov distribution, commonly employed in analysis of positron experiments, is not an accurate representation of the true profile. By comparing results obtained fo different models for the scattering cross sections we identify the transport cross section for elastic scattering and the stopping cross section for inelastic scattering as the parameters characterising the influence of the different scattering processes on the positron transport.


Surface Science | 1992

Measurement of positron annihilation induced Auger electron emission from Fe, Cu, Pd, and Au

K. H. Lee; Ali R. Koymen; D. Mehl; Kjeld O. Jensen; A.H. Weiss

Abstract Positron annihilation induced Auger electron spectroscopy (PAES) makes use of electron-positron annihilation to create core holes which result in Auger electron emission. This excitation process provides PAES with significantly higher surface selectivity than conventional Auger techniques. To date, PAES measurements have been reported on 5 elements. In this paper we report the first positron annihilation measurements of Pd and Au along with measurements of Cu and Fe. Analysis of the data indicates that the intensities of the low energy Auger signals of Cu, Fe, Pd, and Au are in the proportion of 1.00:0.57:1.35:1.17. These measurements should be useful in providing elemental standards for investigations of epitaxial growth and/or intermixing in bimetallic systems. The measured intensities are compared to calculations for core annihilation probabilities. In addition, measurements which demonstrate the sensitivity of PAES to H2 chemisorption on Pd are presented.


Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms | 1991

Positron annihilation induced Auger electron spetroscopy and its implementation at accelerator based low energy positron factories

A.H. Weiss; Ali R. Koymen; D. Mehl; Kuen Ho Lee; G. Yang; Kjeld O. Jensen

Abstract Positron annihilation induced auger electron spectroscopy (PAES) makes use of a beam of low energy positrons to excite Auger transitions by annihilating core electrons. The large secondary electron background usually present in Auger spectra can be eliminated by setting the positron beam energy well below the Auger electron energy. This allows true Auger lineshapes to be obtained. Further, because the positron is localized just outside the surface before it annihilates, PAES is extremely sensitive to the topmost atomic layer. Recent PAES results obtained at the University of Texas at Arlington will be presented. In addition, the use of high resolution energy analyzers with multichannel particle detection schemes to prevent problems due to the high data rates associated with accelerator based positron beams will be discussed.


Surface Science | 1989

Positron trapping at ideal and defected metal surfaces

Ap Brown; Kjeld O. Jensen; Alison B. Walker

Abstract Positrons are becoming increasingly popular as a tool for probing surfaces and are especially sensitive to local electronic structure. Although the idea of positron trapping at ideal surfaces is understood in terms of an image potential induced state which is delocalised in the plane of the surface, little is known about the effect of vacancies and ledges. A theoretical investigation is made here of the annihilation characteristics of positrons trapped at ideal and defected surfaces of aluminium and calculations are compared with recent experimental measurements. Results suggest that positrons may be localised in the surface plane by trapping at vacancies but not at ledges. An important annihilation characteristic is the momentum distribution of the annihilating electron-positron pair with respect to momentum components parallel and perpendicular to the surface. For vacancy trapped states this momentum distribution shows similar isotropy to the experimental results whereas for the delocalised, ideal surface state there is pronounced anisotropy. This seems to indicate that it may be vacancy trapped states which are responsible for the observed isotropy.


Vacuum | 1988

Positron states on perfect and defected aluminium surfaces

Ap Brown; Kjeld O. Jensen; Alison B. Walker

Abstract We report atomistic calculations of positrons trapped in the image potential well at clean Al(110) surfaces. Our results show that, depending on the description of the image potential, it may be energetically favourable for positrons to be localized in the surface plane at monovacancies and multivacancies. However no trapping was observed at ledges. The first calculations of angular correlation spectra for a discrete lattice are presented and compared with recent measurements. We find that the isotropy observed in the experimental ACAR curves cannot be explained by assuming that the positron is trapped at a defect. The overall implications of our work for the interpretation of experimental measurements for positrons at surfaces are discussed.


4th International workshop on: Slow‐positron beam techniques for solids and surfaces | 2008

Positron states on the Cs/Cu(100) surface

Ali R. Koymen; K. H. Lee; D. Mehl; A.H. Weiss; Kjeld O. Jensen

The attenuation of the CuM23VV Auger peak with Cs coverage on Cu(100) is measured using both positron‐annihilation‐induced Auger electron emission (PAES) and conventional (electron induced) Auger electron spectroscopy (EAES). The Cs coverage varies from 0 to 1 physical monolayer (ML). The data indicates that below 0.5 ML in agreement with first order theoretical calculations the positrons are trapped at the Cu/Cs interface. At higher Cs coverages the thermal desorption of the positrons as positronium drops the PAES intensity to zero whereas the EAES signal changes linearly as expected.


4th International workshop on: Slow‐positron beam techniques for solids and surfaces | 2008

Positron thermalisation and nonthermal trapping in aluminium

Kjeld O. Jensen; Alison B. Walker

Results are presented from an investigation of the slowing down and thermalisation of positrons implanted into metals. We have solved the Boltzmann equation for the positron momentum distribution in a homogeneous medium allowing the positrons to scatter off electrons and phonons. We obtained both the time‐dependent and steady‐state solutions. Our results give a more detailed and accurate description of positron thermalisation than earlier work and we show that non‐thermal trapping is not important for positron studies of vacancies in metals.


Physical Review B | 1993

Temperature-dependent top-layer composition of ultrathin Pd films on Cu(100)

Ali R. Koymen; K. H. Lee; G. Yang; Kjeld O. Jensen; A.H. Weiss

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A.H. Weiss

University of Texas at Arlington

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Ali R. Koymen

University of Texas at Arlington

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D. Mehl

University of Texas at Arlington

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K. H. Lee

University of Texas at Arlington

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Ap Brown

University of East Anglia

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G. Yang

University of Texas at Arlington

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A. B. Walker

University of East Anglia

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Ian T. Penfold

University of East Anglia

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