Showing papers on "Positronium published in 1989"
TL;DR: In this article, the positron affinity is defined as the sum of the Fermi level and the bottom of the lowest positron band relative to a common, well-defined reference energy.
Abstract: The relevant quantity in the comparison of the absolute positron energy levels in different materials is the sum of the internal electron and positron chemical potentials, i.e. the sum of the Fermi level and the bottom of the lowest positron band relative to a common, well-defined reference energy. This sum is defined as the positron affinity. The positron affinity reflects the preference of the positron for different components in heterostructures made of different materials and the preference between the host matrix and precipitates in alloys. Moreover, the affinity is closely related to the positron work function and positronium formation potential which are important parameters in the slow-positron-beam experiments. The authors have determined the positron affinity for the alkaline and alkaline-earth metals, 3d-, 4d-, and 5d-transition metal series, and for some metals on the right in the Periodic Table. The diamond structure semiconductors are also considered. The determination is based on the self-consistent electron structure calculations and the subsequent calculation of the positron band structure within the local-density approximation. The trends are studied and interpreted along the different columns and rows of the Periodic Table. The results are also compared with available experiments.
203 citations
TL;DR: In this article, general analytic expressions for the radiative recombination of positrons to form positronium were derived for the capture to bound states of atomic hydrogen, and the connection between the two problems was demonstrated.
Abstract: In the present evaluation of the rate of direct positron annihilation with electrons in the nonrelativistic limit, general analytic expressions are given for the radiative recombination of positrons to form positronium. Formulae are derived for the radiative capture to bound states of atomic hydrogen, and the connection between the two problems is demonstrated. Annihilation from excited states of positronium is considered, and it is estimated that 90 percent of the annihilations occur from the ground 1s state for both ortho and para positronium following radiative capture and cascade. A convenient form is given for the photodissociation cross section of positronium.
56 citations
TL;DR: In this article, a retarding potential time-of-flight method was used to measure the total cross section (TCS) for 0.7-400 eV positrons colliding with acetylene (C2H2) molecules.
Abstract: Total cross section (TCS) for 0.7-400 eV positrons and 1-400 eV electrons colliding with acetylene (C2H2) molecules are measured using a retarding potential time-of-flight method. The results are compared with those in ethylene (C2H4), ethane (C2H6) and methane (CH4) molecules. The effect of the triple bond in C2H2 and the double bond in C2H4 on the TCS curves is explained by the polarisation interaction. The positronium (Ps) formation cross section is also estimated just above threshold.
50 citations
TL;DR: Time-of-flight methods estimate the energy spectrum of the Ps that diffuses out of the target into the vacuum and, as expected, higher implantation energies and lower target temperatures result in a colder Ps energy spectrum.
Abstract: Slow positron pulses are implanted at variable energies into a ${\mathrm{SiO}}_{2}$ powder target to form positronium (Ps). By time-of-flight methods we estimate the energy spectrum of the Ps that diffuses out of the target into the vacuum. As expected, higher implantation energies and lower target temperatures result in a colder Ps energy spectrum. However, even with 19-keV positron implantation energies, the Ps escaping from a target at 4.2 K is only about 2% thermalized. No evidence was found for a low-energy cutoff due to Ps localization between the powder grains.
42 citations
TL;DR: A scheme for a precision measurement of the 1/ital S/-2/ITAL S/ transition frequency in positronium and muonium is reported, and corrections to the two most recent experiments are presented which bring their results into agreement with theory.
Abstract: A scheme for a precision measurement of the 1S-2S transition frequency in positronium and muonium is reported, and corrections to the two most recent experiments are presented which bring their results into agreement with theory.
41 citations
TL;DR: In this article, an integral form of the close-coupling approximation has been employed to investigate positron-hydrogen scattering, and two coupling schemes (1s 2s-2p-Ps and 1s 2 s-2s2pPs) are employed and the present method includes the short- and long-range correlations partially.
Abstract: An integral form of the close-coupling approximation has been employed to investigate positron-hydrogen scattering. Two coupling schemes (1s-2s-2p-Ps and 1s-2s-2p-Ps) are employed and the present method includes the short- and long-range correlations partially. The calculations have been carried out for low (
33 citations
TL;DR: Perturbative corrections to the wave functions of the 16 positronium states were used to derive the {ital n}=1{leftrightarrow} ǫ = 2 radiative transition rates of a positroniam atom moving in arbitrarily oriented electric and magnetic fields as mentioned in this paper.
Abstract: Perturbative corrections to the wave functions of the 16 {ital n}=2 positronium states are used to derive the {ital n}=1{leftrightarrow}{ital n}=2 radiative transition rates of a positronium atom moving in arbitrarily oriented electric and magnetic fields. Excess annihilation radiation resulting from laser excitation and magnetic mixing in {ital n}=2 positronium is calculated and found to be in accord with a recent experiment.
28 citations
TL;DR: As an early non-destructive and in situ testing tool, positron annihilation spectroscopy (PAS) has been utilized extensively to measure crystal defects.
Abstract: The prediction and discovery of the existence of positron (e+)1-4 and positronium atom (Ps)5,6 led rapidly to the development of a new spectroscopic technique for the study of the structure and composition of matter. The technique quickly expanded across interdisciplinary from physics to chemistry, metallurgy, biochemistry and recently to superconductors. As an early non-destructive and in situ testing tool, positron annihilation spectroscopy (PAS) has been utilized extensively to measure crystal defects.7-9
25 citations
TL;DR: Using a monoenergetic, variable-energy positron beam, measurements of Doppler broadening and positronium fraction for positron-annihilation radiation from high-resistivity silicon have been made up to the melting point, showing no clear evidence of positron trapping into thermally generated monovacancies.
Abstract: Using a monoenergetic, variable-energy positron beam, measurements of Doppler broadening and positronium fraction for positron-annihilation radiation from high-resistivity silicon have been made up to the melting point. The measurements show no clear evidence of positron trapping into thermally generated monovacancies which Dannefaer, Mascher, and Kerr observed by measuring the lifetimes of positrons emitted by an {ital in} {ital situ} source. A lower limit of about 3.6 eV is placed on the formation enthalpy of a neutral monovacancy, subject to assumptions about positron-trapping rate, binding energy, and monovacancy formation entropy.
24 citations
TL;DR: Muonium and positronium potentials with radiative corrections are derived from the scattering operator, and general results for the spectra of these systems to order $\ensuremath{\alpha}}^{3}{R}_{\ensure Math{\infty}}$ are obtained.
Abstract: Muonium and positronium potentials with radiative corrections are derived from the scattering operator, and general results for the spectra of these systems to order ${\ensuremath{\alpha}}^{3}{R}_{\ensuremath{\infty}}$ are obtained. We also compare our results with those obtained earlier with the Bethe-Salpeter approach, and find a discrepancy in the case of the $1S$ state. Our theoretical results are in good agreement with the available experimental data.
22 citations
TL;DR: This model of the hyperfine splitting of the muon, the positron, and the proton agrees with the observed values in muonium, positronium, and hydrogen, to within the uncertainty in the respective QED corrections.
Abstract: The nucleus is represented as a Kerr-Newman source, under the assumption that the angular momentum of the source is equal to the intrinsic spin angular momentum of the nucleus. The theoretical values of the hyperfine splitting obtained from a solution of Dirac's equation for this model of the muon, the positron, and the proton agree with the observed values in muonium, positronium, and hydrogen, to within the uncertainty in the respective QED corrections: except for an unaccounted factor of 2. In hydrogen one also has to introduce the observed value of the magnetic moment of the proton. The singularity in Dirac's radial differential equations is shifted from the origin, where it is located in the case of flat space, to a pair of conjugate points on the imaginary axis of the radial coordinate. Consequently, the energy eigenvalues have a trace of an imaginary part, making the bound states of our model unstable.
TL;DR: In this paper, the authors have calculated differential cross sections as a function of the projectile scattering angle for positronhelium and positron-krypton collisions using the classical-trajectory Monte Carlo technique.
Abstract: We have calculated differential cross sections as a function of the projectile scattering angle for positron-helium and positron-krypton collisions using the classical-trajectory Monte Carlo technique. These intermediate-velocity collisions have been simulated by various independent-electron and n-electron models, using both screened Coulomb and quantum model potentials to approximate the effects of electron-electron interactions. These several models all indicate that scattering of positrons to large angles in ionizing collisions persists to high impact velocities. In a previous work [Phys. Rev. A 38, 1866 (1988)] we proposed that the recent experiments that have measured the total cross section for positronium formation could be affected by the loss of positron flux due to incomplete confinement after large-angle scattering. Here, utilizing these newly calculated differential-scattering cross sections, adjustments are computed that account for the difference between the experimental and theoretical behaviors of the positronium formation cross section. Further, it is demonstrated that large-angle scattering in the elastic channel is important for one of the experiments. We have also computed total cross sections for ionization and charge transfer for collisions of protons with krypton. Good agreement is obtained in these cases with experimental measurements.
TL;DR: The three-body Schroedinger equation is solved directly for the ground state of the positronium negative ion by using a rapidly convergent correlation function hyperspherical harmonic method, which involves no adjustable parameters.
Abstract: The three-body Schr\"odinger equation is solved directly for the ground state of the positronium negative ion by using a rapidly convergent correlation function hyperspherical harmonic method, which involves no adjustable parameters. The inclusion of 169 hyperspherical functions yields the ground-state energy 0.262 004 86 a.u., which is converged to an error in the seventh significant figure. Expectation values of different functions of interparticle distances as well as the two-photon annihilation rate are calculated and compared with those obtained by variational calculations.
TL;DR: In this article, the authors have experimentally detected time oscillations of the γ-quantum angular distribution under positronium annihilation in a magnetic field, based on this phenomenon it is possible to realize a new method for studying matter with the use of positrons.
TL;DR: The method of complex-coordinate rotation is applied to investigate autoionization states of positronium molecules to determine resonance parameters for six S-wave resonant states, believed to belong to a Rydberg series.
Abstract: The method of complex-coordinate rotation is applied to investigate autoionization states of positronium molecules. Using elaborate Hylleraas-type wave functions in which all six interparticle coordinates are used, resonance parameters for six S-wave resonant states are determined. It is believed that three of them belong to a Rydberg series in which the two positrons form a singlet spin state, and the other three resonances belong to the triplet spin state for the two positrons. All these two Rydberg series are converging on the ground state of the Ps/sup -/ threshold.
TL;DR: In this paper, it was shown that while detecting positronium 3 gamma-decay quanta in a magnetic field one may observe counting rate oscillations at a frequency of triplet PLE splitting.
Abstract: It is shown that while detecting positronium 3 gamma -decay quanta in a magnetic field one may observe counting rate oscillations at a frequency of triplet positronium level splitting. The oscillations may be observed in detecting the decay quanta by both three or one (two) counters. Based on this phenomenon it is possible to realise a novel matter research method, similar to the well known muonium spin rotation method.
01 Sep 1989
TL;DR: In this paper, a new kinetic model involving the reversible formation of a Ps complex with solute S, [PsS], as an intermediate step toward Ps oxidation or spin conversion was proposed.
Abstract: The quenching reactions of positronium with the diamagnetic cobalt and with the paramagnetic chromium acetylacetonates have been studied in benzene solutions at various temperatures by both lifetime spectroscopy and the Doppler broadening of the annihilation radiation lineshape technique, DBARL. Both solutes inhibit Ps formation and quench its lifetime, respectively by redox and by spin conversion reactions for the Co(III) and Cr(III) compounds. The decrease upon heating of the overall apparent quenching rate constants and the Doppler data are both well explained on the basis of a new kinetic model involving the reversible formation of a Ps complex with solute S, [PsS], as an intermediate step toward Ps oxidation or spin conversion.
TL;DR: In this article, it was shown that for the coupling required to explain ge−2 and positronium, a mass of at least 34 keV (175 keV) is required.
TL;DR: In this paper, the separate ingredients known to be important in positronium formation, such as band structure, surface effects, matrix elements, scattering of the incoming positron wave and higher order contribution due to interaction of positrons with a remaining hole at the surface beyond the golden rule are drawn together within a single dynamic formalism with the objective of finding a rapid method of calculation.
TL;DR: In this paper, the basic physical fear of positron annihilation (PA) is discussed and a biref smmary of the basic physics fear of PA is presented, together with representative applications of PA from the fields of physics, chemistry and medicine.
Abstract: A biref smmary of the basic physical fearutes of positron annihilation (PA) is presented. The information content of PA, the fate of positrons in condensed media, the formation of positron bound states and positronium (Ps) are considered in connection with the experimental techniques of PA. Representative applications of PA from the fields of physics, chemistry and medicine are given to illustrate the virtue of these techniques. Performance and sensitivity limits and the future perspectives of PA are also briefly discussed.
TL;DR: In this article, a new explanation is proposed of the anomalous magnetic quenching of positronium found by others for solutions of nitrobenzene in hexane, but not benzene and cyclohexane.
TL;DR: The positronium formation processes in the ground and excited n = 2 levels have been investigated in e/sup +/-H atom collision using the Glauber eikonal approximation and results are found to differ appreciably from the corresponding first-order Born values even at very high energies.
Abstract: The positronium formation processes in the ground and excited n = 2 levels have been investigated in e/sup +/-H atom collision using the Glauber eikonal approximation. Both the differential and total formation cross sections have been studied in the intermediate- and high-energy regime. Present eikonal results are found to differ appreciably from the corresponding first-order Born values even at very high energies. The total-cross-section results have been compared with other existing theoretical values as well as with the corresponding Born cross sections.
TL;DR: The magnetic quenching of the triplet state of positronium (o•Ps) was studied in pure n•hexane at 294 K, by applying an external magnetic field.
Abstract: The magnetic quenching of the triplet state of positronium (o‐Ps) has been studied in pure n‐hexane at 294 K, by applying an external magnetic field. The measured annihilation parameters are characteristic of a normal second‐order Zeeman effect and betray some slight expansion of the Ps wave function as compared to in vacuo. In the presence of nitrobenzene, which reacts with Ps to form a complex, very strong effects are observed, confirming previous experiments. The lifetime of the o‐Ps (m=0) substate undergoes a drastic decrease, even at very low magnetic field strengths. A new approach to this unexpected phenomenon is proposed, involving the reversible formation of a weak complex between Ps and the solute. Various quantitative treatments are attempted to fit the data. As it seems, the data can only be described if one supposes that there is no spin reorientation during the reaction. In all cases, the derived fitting parameters are characteristic of the complex, therefore denoting that the field effect d...
TL;DR: Energy and angular distributions of the positronium emitted from single-crystal samples of the alkali halides LiF, NaF, KCl, KBr, and KI following bombardment of the surfaces with 0.1--4-keV positrons are reported.
Abstract: Energy and angular distributions of the positronium emitted from single-crystal samples of the alkali halides LiF, NaF, KCl, KBr, and KI following bombardment of the surfaces with 0.1--4-keV positrons are reported. Energetically narrow positronium distributions were formed at the positron implant depth, and this hot positronium diffused to the surface, losing energy in the process. An analysis of the data using a diffusion model yields an estimate of the strength of the positronium-phonon coupling. This coupling is found to be energy dependent, with the effective deformation potential decreasing as the positronium energy increases. Positronium work functions and binding-energy estimates are also given. The angular distributions of the emitted positronium from all samples except LiF were sharply peaked in the forward direction. The energy distributions from the LiF samples also differed from those of the other alkali halides by showing two distinct energy components instead of a single peak.
TL;DR: In this article, the effects of incomplete positron thermalization and uncertainties of the positron implantation profile at low-positron incident energies to the measured data are discussed.
Abstract: In this paper we present and discuss experimental methods to determine the positron diffusion coefficient from slow positron beam measurements. We also evaluate the use of the annihilation line Doppler-broadening technique in positron diffusion measurements, as compared on the more commonly used method of positronium fraction. The effects of incomplete positron thermalization and uncertainties of the positron implantation profile at low-positron incident energies to the measured data are discussed. We apply the presented methods to the model case of A1(110) system in the temperature range from 20 to 500 K. This data shows that Doppler-broadening and positronium fraction measurements give consistent results for the positron diffusion coefficient in A1(110), where D+(300 K) = 1.7(2) cm2/s with the temperature dependence D+ ∝T
−0.62(3).
TL;DR: In this paper, the problem of binding in positron-negative ion systems has been addressed via two-component density functional theory, and the results indicate that a positron forms a stable bound state with the negative ions Li−,B−,C−,O−,F− and Cl− with respect to dissociation into a negative ion and either a neutral atom and positronium.
TL;DR: The formation of positronium in different states by the impact of intermediate energy (20--500 eV) positrons on atomic hydrogen is calculated using the distorted-wave Born approximation along with the first Born approximation.
Abstract: The formation of positronium (Ps) in different states by the impact of intermediate energy (20--500 eV) positrons on atomic hydrogen is calculated using the distorted-wave Born approximation along with the first Born approximation. Differential and integrated cross sections for the formation of Ps(1{ital s}), Ps(2{ital s}), Ps(2{ital p}{sub 0}), and Ps(2{ital p}{sub 1}) have been evaluated individually and then 1/{ital n}{sup 3} behavior for charge-transfer cross sections is used for {ital n}{gt}=3 to obtain the total cross sections for positronium formation. The present results for positronium formation cross sections are in agreement with the results of other available investigations.
TL;DR: Measurements using the improved two-dimensional angular correlation of annihilation radiation technique on surfaces support three established processes: spontaneous positronium (Ps) formation and emission, positrons bound in a surface state, and surface-state positrons thermally desorbed as Ps atoms.
Abstract: Studies were made of the interactions of positrons with three low index surfaces of aluminum [(100), (110), and (111)] at room temperature with the aluminum surface at elevated temperatures and with oxygen adsorption on the aluminum surface. Our measurements using the improved two-dimensional angular correlation of annihilation radiation technique on surfaces support three established processes: spontaneous positronium (Ps) formation and emission, positrons bound in a surface state, and surface-state positrons thermally desorbed as Ps atoms. A method has been developed to accurately separate these components. The positron surface-state annihilation spectra are nearly isotropic for all three Al surfaces, which indicates a lateral localization and is inconsistent with either the ideal image-potential-induced bound state or the physisorbed Ps state. The thermal Ps momentum distributions agree well with the thermostatistical emission mechanism. The momentum distributions of the directly emitted Ps atoms are found to be very sensitive to the surface conditions, and to reflect the electron density of states near the surfaces, thus suggesting a new surface spectroscopy, angle-resolved Ps spectroscopy.
TL;DR: The polarization effect of a magnetic field on the positronium components in the one-dimensional angular correlation of annihilation radiation in KI has been investigated and it has been confirmed that practically all positronia atoms annihilate from the free states at 15 K.
Abstract: The polarization effect of a magnetic field on the positronium components in the one-dimensional angular correlation of annihilation radiation in KI has been investigated. It has been confirmed that practically all positronium atoms annihilate from the free states at 15 K. Rapid increase in the fraction of the positronium atoms annihilating from the self-trapped state with increasing temperature is also confirmed. An interpretation of the results is given in terms of the metastable self-trapping of positronium suggested previously. The self-trapping of positronium is discussed in connection with that of excitons.
TL;DR: In this article, the authors review the physics of the negative affinity and hot-positron moderators that allow us to produce intense beams of slow positrons by repeated staes of remoderation.
Abstract: We review the physics of the negative affinity and hot-positron moderators that allow us to produce intense beams of slow positrons. By repeated staes of remoderation a positron beam can be focused to a microscopic spot on a solid target with little loss of intensity. The positronium (Ps) and positronium negative ions (Ps−) produced by such a spot will be bright enough to permit the formation of well-collimated Ps beams suitable for gravitational free fall and other experiments.