Showing papers on "Positronium published in 1980"
TL;DR: In this paper, the brightness of slow positron beams can be enhanced significantly by repeated stages of moderation, acceleration and focusing, which can be used for characterizing surfaces by positron diffraction and microscopy.
Abstract: The brightness of slow positron beams can be enhanced significantly by repeated stages of moderation, acceleration and focusing. Presently available data suggest that the source spot area should decrease by 10−4 after each stage with only a modest loss of intensity. Beams with very small angular divergence, which could be made with this technique, would be useful for characterizing surfaces by positron diffraction and microscopy. Using such beams it is possible to envision the study of new exotic systems such as thee
+-e
− plasma and the positronium molecule.
129 citations
TL;DR: In this article, a range of metallic surface processes available to the implanted positron is examined with a view to bare positron escape, positron surface state trapping, surface state detrapping, and positronium atom conversion.
Abstract: The range of metallic surface processes available to the implanted positron is examined with a view to bare positron escape, positron surface state trapping, surface state detrapping, and positronium atom conversion. The stopping distance and its rms deviation are calculated. The effect of the elastic positron-ion core scattering, and the finite temperature corrections in the positron-phonon scattering are included. Overall conversion and emission efficiencies are calculated by solving an appropriate diffusion equation with boundary conditions set by the branching ratios for the surface processes. The temperature and incident energy dependence of the results for positronium atom conversion and positron emission efficiency are compared to experimental data.
96 citations
TL;DR: In this article, total scattering cross sections have been measured in the same apparatus for positrons and electrons of well-defined energies (0.35-100 eV) colliding with krypton and xenon using a beam-transmission technique.
Abstract: Total scattering cross sections have been measured in the same apparatus for positrons and electrons of well-defined energies (0.35--100 eV) colliding with krypton and xenon using a beam-transmission technique. The positron total cross sections are largest at the lowest energies studied, and decrease in a manner very similar to the elastic scattering cross-section calculations by Schrader and by McEachran et al. The total cross sections for positron-krypton, xenon collisions experience abrupt changes in their slope at the respective positronium-formation thresholds indicating that positronium formation may be an important contributor to the total cross section above the formation threshold. The present electron measurements are in good agreement with the recent measurements of Wagenaar. Estimates of potential errors in the present measurements due to incomplete discrimination against small-angle forward elastic scattering are made.
92 citations
Journal Article•
TL;DR: In this article, a range of metallic surface processes available to the implanted positron is examined with a view to bare positron escape, positron surface state trapping, surface state detrapping, and positronium atom conversion.
Abstract: The range of metallic surface processes available to the implanted positron is examined with a view to bare positron escape, positron surface state trapping, surface state detrapping, and positronium atom conversion. The stopping distance and its rms deviation are calculated. The effect of the elastic positron-ion core scattering, and the finite temperature corrections in the positron-phonon scattering are included. Overall conversion and emission efficiencies are calculated by solving an appropriate diffusion equation with boundary conditions set by the branching ratios for the surface processes. The temperature and incident energy dependence of the results for positronium atom conversion and positron emission efficiency are compared to experimental data.
83 citations
TL;DR: In this article, a simple positron diffusion model including surface and vacancy trapping was fitted with a simple diffusion model, assuming that positronium is formed only at the surface, and the formation of part of the positronion fraction was identified as detrapping from a surface state, and an estimate of the binding energy in this trap was deduced.
Abstract: Monoenergetic positrons, with an incident energy of 0-5 keV, were focused onto Ag(100), Ag(111), and Cu(111) surfaces with submonolayer contamination, and positronium formation was studied as a function of sample temperature from 300 to 1200 K. The data were fitted with a simple positron diffusion model including surface and vacancy trapping, assuming that positronium is formed only at the surface. The formation of part of the positronium fraction is found to be a temperature-activated process which is identified as detrapping from a surface state, and an estimate of the binding energy in this trap is deduced. The diffusion length is found to be only slightly temperature dependent between room temperature and the onset of vacancy trapping. At the higher sample temperatures positron trapping at thermally generated vacancies is observed by the decrease in the positron diffusion length at the higher incident voltages. A vacancy formation energy is extracted from the data and is generally found to be lower than the values obtained from bulk measurements.
78 citations
TL;DR: In this paper, the elastic scattering of positrons from noble gases has been formulated in a frozen core version of the polarised-orbital approximation, which has been applied to the scattering of POSs from krypton and xenon.
Abstract: The elastic scattering of positrons from noble gases has been formulated in a frozen-core version of the polarised-orbital approximation. This approximation has been applied to the scattering of positrons from krypton and xenon. The authors present results for phaseshifts and elastic, diffusion and annihilation cross sections in the low-energy (elastic scattering) region, as well as the angular correlation of the photons produced during annihilation. Good agreement is obtained between the elastic scattering cross section and recent experimental results for xenon below the positronium formation threshold. The agreement is less satisfactory in the case of krypton.
77 citations
TL;DR: In this article, a simple positron diffusion model, including surface and vacancy trapping, is fitted to the positronium data and an estimate of the binding energy of a positron in this trap is made.
Abstract: Well-characterized Al(111) and Al(100) samples were studied with monoenergetic positrons before and after exposure to oxygen. Both positronium-formation and positron-emission curves were obtained for various incident positron energies at sample temperatures ranging from 160-900 K. The orthopositronium decay signal provides a unique signature that the positron has emerged from the surface region of a clean metal. In the clean Al crystals part of the positronium formed near the surface is found to be associated with a temperature-activated process described as the thermally activated detrapping of a positron from a surface state. A simple positron diffusion model, including surface and vacancy trapping, is fitted to the positronium data and an estimate of the binding energy of the positron in this trap is made. The positron diffusion constant is found to have a negative temperature dependence before the onset of positron trapping at thermally generated monovacancies (g500 K), in reasonable agreement with theoretical predictions. The depth of the positron surface state is reduced or positronium is formed in the chemisorbed layer as oxygen is adsorbed on both Al sample surfaces, thus increasing the positronium fraction and decreasing the positron emission. At higher oxygen exposures [g500 L (1 L = ${10}^{\ensuremath{-}6}$ torr sec)] positron or positronium traps are generated in the overlayer and the positronium fraction is reduced. The amorphous-to-crystalline surface transition of ${\mathrm{Al}}_{x}{\mathrm{O}}_{y}$ on Al is observed between 650 and 800 K by the change in the positronium fraction and is interpreted as the removal of trapping centers in the metal-oxide overlayer. At the higher temperatures and incident energies vacancy trapping is observed by the decrease in the positron diffusion length in both the clean and the underlying Al of the oxygen-exposed samples. Similar vacancy formation enthalpies for Al are extracted in both the clean and oxygen-covered samples by a simple model and are in good agreement with those measured by other experimental methods. This technique provides a new experimental means for the study of interfaces and thin films and the vacancy-type defects associated with them.
52 citations
TL;DR: In this article, the Doppler broadened positron annihilation line shapes have been determined in aqueous solutions of various inhibiting and enhancing agents of positronium (Ps) formation: H+aq, NO−3, S2−, SCN− and the halide ions.
Abstract: The Doppler broadened positron annihilation line shapes have been determined in aqueous solutions of various inhibiting and enhancing agents of positronium (Ps) formation : H+aq, NO−3, S2−, SCN− and the halide ions. The results confirm that the inhibition is made through electron scavenging by H+aq and NO−3, and through positron capture, to form a bound state, by the other ions. The characteristics and yields of the various positron‐containing species in the solutions are inferred from the deconvoluted experimental line shapes. On referring to the lifetime spectroscopy experimental data, it is found that the yields of positron bound states are lower than expected, and the formation of excited states is hypothesized to account for the discrepancy.
46 citations
TL;DR: In this article, a pseudopotential formalism was used to predict the cross section for positronium formation during collisions of positrons with the alkali atoms Na, K, Rb and Cs employing the first Born approximation and the distorted wave approximation.
Abstract: Calculations are performed within the scope of the pseudopotential formalism to predict the cross section for positronium formation during collisions of positrons with the alkali atoms Na, K, Rb and Cs employing the first Born approximation and the distorted-wave approximation. This maiden attempt allows the authors to obtain an idea of the order of magnitude of the positronium formation cross section for these atoms. Retaining both the post and the prior forms of the interaction potential in the calculation, results for the differential, total and momentum-transfer cross sections are computed at several incident positron energies in the range 0.5-20 eV.
31 citations
TL;DR: In this paper, the formation energy of the defects which trap ortho-positronium is estimated at E f v = 70 ± 10 kJ mol -1 (0.73 ± 0.1 eV).
30 citations
TL;DR: In this article, the positron motion is described by a one-dimensional diffusion equation, and the measurement of vacancy activation energies, positron diffusion constants, and positron stopping profiles are discussed.
Abstract: Low-energy positrons implanted into a crystal can diffuse to the surface where they may be detected, for instance, by observing the formation of positronium. We describe the positron motion by a one-dimensional diffusion equation, and discuss the measurement of vacancy activation energies, positron diffusion constants, and positron stopping profiles.
TL;DR: In this paper, the density-functional method is applied to excess electrons and positronium atoms in helium fluids, and the self-trapping is investigated in a fully selfconsistent fashion.
Abstract: The density-functional method is applied to excess electrons and positronium atoms in helium fluids. The self-trapping is investigated in a fully self-consistent fashion, and formulas are given for the particle energy and Ps pick-off annihilation rate in quasifree as well as localized states. The numerical results compare well with experimental data. However, the need for a more sophisticated treatment of threshold effects near the onset of bubble formation is indicated.
TL;DR: In this paper, the authors measured the energy dependence of the positronium formation cross section for the gases He, Ar, H2 and CH4 by passing a beam of slow positrons through a scattering chamber and detecting the POS by counting the triple coincidences from the 3 gamma decay of ortho-positronium.
Abstract: The energy dependence of the positronium formation cross section has been measured for the gases He, Ar, H2 and CH4. This has been accomplished by passing a beam of slow positrons through a scattering chamber and detecting the positronium formed by counting the triple coincidences from the 3 gamma decay of ortho-positronium.
TL;DR: In this article, cross sections for positronium formation into an arbitrary excited s state during positron scattering from the alkali atoms using the first Born approximation were computed for the incident positron energies from 5 to 50 eV.
Abstract: For pt.I see ibid., vol.13, no.9, p.1919 (1980). Cross sections are computed for positronium formation into an arbitrary excited s state during positron scattering from the alkali atoms using the first Born approximation. Considering both the post and the prior forms of the interaction potential in the calculation, the results are presented for the incident positron energies from 5 to 50 eV. From these results it is found that for the alkali atoms the Ps formation cross section satisfies the inverse n3 law consistently at all positron energies.
TL;DR: In this article, a systematic perturbation theory for the computation of positronium energy levels is presented, based on the Bethe-Salpeter equation and a zeroth-order kernel for which exact solutions are known explicitly.
TL;DR: The positron annihilation data also sensitively reflect structural rearrangements in these solutions occurring upon further addition of water, such as the transition of spherical aggregates to a disk-like lamellae structure as discussed by the authors.
Abstract: The formation of thermalized positronium atoms is greatly reduced if increasing amounts of water become solubilized in reversed micelles formed by sodium bis(2-ethylhexyl) sulfosuccinate in apolar solvents. Similar observations have been made if the surfactant is Triton X-100. The application of the positron annihilation technique to the study of microemulsions consisting of potassium oleate-alcohol-oil-water mixtures indicates, consistent with previous results, that microemulsion formation requires a certain water/oil ratio if the oil is a long-chain aliphatic hydrocarbon such as hexadecane. This ratio is 0.4 in the case of a 1-pentanol- and 0.2 for a 1-hexanol-containing mixture. This minimum water content is strongly reduced if the oil is an aromatic hydrocarbon. The positron annihilation data also sensitively reflect structural rearrangements in these solutions occurring upon further addition of water, such as the transition of spherical aggregates to a disk-like lamellae structure.
TL;DR: Positronium yields as a function of pyridine concentration were studied in nine nonpolar solvents as mentioned in this paper, and strong enhancements in the Ps yields on addition of low concentrations (
TL;DR: In this paper, it was shown that the molecular effects play a significant role in the case of positronium formation and a hydrogen molecule cannot be considered as a pair of independent hydrogen atoms at any energy.
Abstract: Positronium formation in the ground state and in a number of excited s states including the asymptotic n to infinity has been studied for the reaction e++H2 to Ps(ns)+H2+ in the molecular Jackson-Schiff approximation (1953) in the energy range 50-1000 eV. It is found that the molecular effects play a significant role in the case of Ps formation and a hydrogen molecule cannot be considered as a pair of independent hydrogen atoms at any energy. The excited-state formation cross sections satisfy an n-3 law approximately. It is also found that Ps formation involving the gerade transition of H2++ is the dominant process, although depending on the choice of molecular wavefunction there is a 10-28% probability of Ps formation involving the ungerade transition.
TL;DR: The scattering amplitude for positronium formation into arbitrary excited S states in positron-helium collisions in which the residual helium ion is left in the ground state, is obtained in a closed form using the first Born approximation and the first-order exchange approximation as discussed by the authors.
Abstract: The scattering amplitude for positronium formation into arbitrary excited S states in positron-helium collisions in which the residual helium ion is left in the ground state, is obtained in a closed form using the first Born approximation and the first-order exchange approximation. Retaining both the post and the prior forms of the interaction potential, cross sections are computed for several incident positron energies in the range 20 --2000 eV. For these energies the asymptotic limit of n/sup 3/ times the cross section as n ..-->.. infinity is attained roughly within 0.1% beyond the excited state 16S.
TL;DR: In this article, it was shown that there exists a metastable self-trapped state of positronium at about 0.1 eV above the ground state of free positRONium.
Abstract: Recently observed anomalous temperature dependence of the width of the positronium peak in NaF is interpreted in terms of metastably self-trapped positronium. Analysis of the data indicates that there exists metastable self-trapped state of positronium at about 0.1 eV above the ground state of free positronium.
TL;DR: In this paper, the temperature dependence of the narrow peaks originating from the self-annihilation of singlet positronium (Ps) is analyzed at temperatures ranging from 4.5 K to 300 K.
Abstract: Positron annihilation 2γ angular correlation curve of NaF has been measured at temperatures ranging from 4.5 K to 300 K. The temperature dependence of the narrow peaks originating from the self-annihilation of singlet positronium (Ps) is analysed. In the lower temperature region, the peak width increases in a rate that shows the Ps is in free state with the effective mass 1.5±0.2 times heavier than the bare mass. At temperatures higher than 150 K, tlae peak width increases more rapidly than in the lower termperature region, which indicates that a number of the Ps are trapped in some localized state.
TL;DR: In this article, a one-step thermal activation model was applied to the data, yielding activation energies of 0.44±0.04, 0.46± 0.04 and 0.75 ± 0.05 eV for Al(100), Ag(polycrystalline), and Ni(100) respectively.
Abstract: Positronium (Ps, a positron–electron bound pair) emission as a result of slow positron (30–1000 eV) bombardment of Cu, Al, Ni, and Ag surfaces has been investigated as a function of incident positron energy, sample temperature, and surface contamination. The fraction of 30 eV positrons that were reemited as Ps at 380 K with <5% surface contamination ranged from 0.43 to 0.47. The emission fraction is found to increase with both increasing surface contamination and incident positron energy. Both thermally activated and nonthermally activated Ps emission is suggested by the data. A one‐step thermal activation model applied to the data yields activation energies of 0.44±0.04, 0.46±0.04 and 0.75±0.05 eV for Al(100), Ag(polycrystalline), and Ni(100), respectively.
TL;DR: In this article, the authors observed that the appearance of a maximum in the positronium (Ps) rate constant Ks and the possibility of the electron transfer is due to thermal fluctuations in the solvent.
Abstract: One of the remarkable effects observed with a change in temperature is the appearance of a maximum in the positronium (Ps) rate constant Ks and (2) the possibility of the electron transfer is due to thermal fluctuations in the solvent. At high temperatures the Kobsd value is determined by the probability of the electron tunneling from Ps to Ac. With the increasing temperature the probability value tends to zero because of the following dependence of the reorganization energy Ereorg on temperature 5": Ereox - F (n > 1). In contrast, at low temperatures Kobd is controlled by diffusion. The rise of T(K,,,) with the increase of the solvent polarity is explained by the decrease of energy required to separate the reaction products to infinity.
TL;DR: In this paper, the effect of static electric field on the formation and annihilation of positronium in polyethylene, polystyrene, teflon, quartz, nylon and perspex has been investigated employing a fast timing spectrometer with prompt resolution (fwhm) ∼200 ps at the positron experimental settings.
Abstract: The effect of static electric field on the formation and annihilation of positronium in polyethylene, polystyrene, teflon, quartz, nylon and perspex has been investigated employing a fast timing spectrometer with prompt resolution (fwhm) ∼200 ps at the positron experimental settings. No significant variation in the lifetime of the long lived pick-off component with electric field has been observed. The intensity of the long-lived pick-off component decreases with increase of the field in all the cases investigated except in nylon and perspex where the intensity remains constant with the field. The results have been discussed in the frame work of the spur reaction model proposed by Mogensen and also in the light of the Ore gap model, as discussed by Brandt and Wilkenfeld. Another interesting correlation seems to exist between positronium formation and dielectric loss factor; in case of increasing loss factor there is a decrease in the effect of electric field.
TL;DR: In this paper, the process for thermalized positrons deposited in a metal by a slow-positron beam was examined, including diffusion back to the surface, trapping in vacancies and surface states, and formation in the surface region.
TL;DR: In this paper, a perturbation theory for positronium energy levels based on the Bethe-Salpeter equation was investigated, and an analytic expression for the sum over intermediate states was obtained.
Abstract: We investigate a perturbation theory for positronium energy levels, based on the Bethe-Salpeter equation. An analytic expression for the sum over intermediate states is obtained. Its application is illustrated by computing a second-order contribution to positronium hyperfine splitting.
01 Jul 1980
TL;DR: Positron annihilation investigations have been performed in several different organic nonpolar solvents, containing different forms of bromine substituted benzenes, and results have been discussed in terms of the "spur theory" and plausible reaction pathways for the difference in inhibitor capacity among the solutes.
Abstract: Positron annihilation investigations have been performed in several different organic nonpolar solvents, containing different forms of bromine substituted benzenes. All solutes behaved as typical inhibitors, although the inhibitor capacity for some of them was drastically decreased upon changing the solvent. Results have been discussed in terms of the “spur theory”, Plausible reaction pathways for the difference in inhibitor capacity among the solutes have been suggested.
TL;DR: In this article, the electron scavenging properties of aqueous solutions of two series of solutes are investigated, using the positron as a probe, using both lifetime spectroscopy and the Doppler broadening of annihilation line shape technique.