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Showing papers on "Positronium published in 1990"


Journal ArticleDOI
TL;DR: Positron annihilation spectroscopy for chemical analysis (PASCA) has been developed to analyze the microstructure of polymers as discussed by the authors, where the positronium atom is trapped in the atomic-scale holes which have a size ranging from 1 to 10 A. Theoretical aspects, experimental developments, and examples in analyzing the microstructures of epoxy, PEEK, and PMMA polymers are presented.

694 citations


Journal ArticleDOI
TL;DR: In this paper, the behavior of the cross-sections for antihydrogen formation in collisions of antiprotons with excited state positronium is discussed and relevant scaling laws are presented.

112 citations


Journal ArticleDOI
TL;DR: A review of low-energy positron beam experiments can be found in this article, where elastic scattering, positronium formation, excitation, ionisation and attachment are discussed.
Abstract: Atomic collision experiments using low-energy positron beams have been actively pursued for more than two decades With greater understanding of the basic physics governing the production of these beams, advances in technology have promoted studies beyond measurements of total collision cross sections Such work forms the basis of this review and includes topics from elastic scattering, positronium formation and the associated production of positronium beams, excitation, ionisation and attachment

64 citations


Journal ArticleDOI
TL;DR: In this article, the angle-integrated ionization cross section for both positron and electron impact on atomic hydrogen (excluding positronium formation) has been measured in the energy region of 17.5 to 600 eV.
Abstract: In a crossed-beam experiment the angle-integrated ionization cross section for both positron and electron impact on atomic hydrogen (excluding positronium formation) has been measured in the energy region of 17.5 to 600 eV. Below 450 eV the positron cross section is larger than the electron one, at 50 eV by about a factor of 2. Absolute cross sections are obtained by comparing the electron data with literature values. At energies above 30 eV all theoretical predictions lie below our measured cross sections for positron-impact ionization.

60 citations


Journal ArticleDOI
TL;DR: The first excitation of high-{ital n} states of positronium is performed using resonant, two-photon excitation applied to the {ital n}{sup {minus}3} scaling of the relative transition rates.
Abstract: We have performed the first excitation of high-n states of positronium using resonant, two-photon excitation applied to the n=1 to 2 and 2 to n transitions. Absolute values for the line positions were quantitatively determined for n=14 and 15 and compared well with calculated predictions. The prediction of the ${\mathit{n}}^{\mathrm{\ensuremath{-}}3}$ scaling of the relative transition rates was observed for n=12 to 19.

54 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used the close-coupling approximation (CCA) to investigate positronium formation into the n=1 and n=2 levels for the positron-hydrogen system.
Abstract: The close-coupling approximation (CCA) has been used to investigate positronium formation into the n=1 and n=2 levels for the positron-hydrogen system. The solution has been obtained by solving integral Lippmann-Schwinger equations in momentum space. The authors have used two basis sets (H(1s,2s,2p)+Ps(1s,2s,2p) and H(1s,2s,2p)+Ps(1s,2s,2p)), which make some allowance for short- and long-range correlation effects in both the incident and positronium channels. These are the first close-coupled results for Ps(n=2) formation. The effect of adding polarization in the positronium channel is found to be significant for Ps formation. A new feature of the present calculations is the use of Gaussian basis functions in the evaluation of the rearrangement matrix elements.

52 citations


Journal ArticleDOI
TL;DR: The positron annihilation rate of positronium negative ions was calculated as 2.08613 ns−1 by using a 946 term Hylleraas-type wave function.

35 citations


Journal ArticleDOI
TL;DR: In this article, the radiative corrections ≈ α 2 ln α to the positronium decay rate were calculated in the Breit approximation which is shown to be quite adequate for the problem.

34 citations


Journal ArticleDOI
TL;DR: An accurate calculation for (1,3)P autodetaching resonances in Ps(-) has been carried out using the complex-rotation method, which has the advantage of giving resonance position and width at the same time.
Abstract: An accurate calculation for (1,3)P autodetaching resonances in Ps(-) has been carried out using the complex-rotation method, which has the advantage of giving resonance position and width at the same time. The wave function is of the Hylleraas type with an available number of terms up to 1140. One 1P and two 3P resonances below the n = 2 threshold and two 1P and two 3P resonances below the n = 3 threshold of the positronium atom. Resonance parameters are compared with those obtained from scattering and adiabatic calculations where available.

31 citations


Journal ArticleDOI
TL;DR: In this paper, high-resolution 2 gamma angular correlation method was used for the conservation of ortho-positronium in oxygen, and the existence of an inelastic conversion process was confirmed.
Abstract: Conservation of ortho-positronium in oxygen has been investigated by the high-resolution 2 gamma angular correlation method. The oxygen pressure was low enough to suppress possible conversion of para-positronium to ortho-positronium. Silica aerogel was used to form a sufficient number of positronium atoms in a restricted region necessary for the high-resolution angular correlation measurements. The existence of an inelastic conversion process has been reconfirmed. The cross sections for the elastic and inelastic conversion processes have been estimated to be (1.0+or-0.3)*10-19 cm2 and some 10-17 cm2, respectively.

29 citations


Journal ArticleDOI
TL;DR: In this article, the authors reported optical saturation of the 13S-23P transition in positronium using ultraviolet light from a frequency-doubled, pulsed dye laser.
Abstract: The authors report optical saturation of the 13S-23P transition in positronium using ultraviolet light from a frequency-doubled, pulsed dye laser. The increase in ground-state annihilation radiation due to magnetic triplet-singlet mixing in the excited state is used to monitor the transition. From the variation of the annihilation rate as a function of magnetic field, laser power, wavelength and polarisation, the authors verify that the transition is optically saturated, in agreement with model calculations of the system.

Journal ArticleDOI
TL;DR: In this paper, the angular correlation technique of positron annihilation has been used to detect and measure the localized bubble state of positronium (Ps) in liquid Ne, Ar, Kr, H2, and N2 and in liquid and solid He at various pressures and temperatures.
Abstract: The angular-correlation technique of positron annihilation has been used to detect and measure the localized bubble state of positronium (Ps) in liquid Ne, Ar, Kr, H2, and N2 and in liquid and solid He at various pressures and temperatures. No bubble state was seen in liquid O2 or in solid Ne and Ar. The dynamics of bubble formation is not yet understood. In the cases where theoretical calculations, and adequate data, exist, viz. He, Ar, and H2, there is reasonable agreement for the momentum of the photons from the annihilation of positrons with the outer electrons of these atoms. The Ps annihilations from the self-trapped bubble state are reasonably well described in terms of a simple finite potential-well model.

Journal ArticleDOI
TL;DR: The frequencies of four $^{130}\mathrm{Te}_{2}$ transitions at 486 nm, which are of interest as references in precision measurements of the 1S-2S transitions in hydrogen, deuterium, and positronium, are determined by interferometric comparison with standard transitions.
Abstract: The frequencies of four $^{130}\mathrm{Te}_{2}$ transitions at 486 nm, which are of interest as references in precision measurements of the 1S-2S transitions in hydrogen, deuterium, and positronium, are determined with an accuracy of 1 part in ${10}^{9}$ by interferometric comparison with $^{127}\mathrm{I}_{2}$ standard transitions.

Journal ArticleDOI
TL;DR: The feasibility of the application of the angular-correlation method to positron spin polarimetry is discussed and asymmetric field dependences of the formation probabilities of both perturbed orthopositonium and perturbed parapositronium are analyzed.
Abstract: Formation of positronium in porous silica glass by spin-polarized positrons in the presence of a magnetic field is investigated using a one-dimensional angular-correlation apparatus. Asymmetric field dependences of the formation probabilities of both perturbed orthopositronium and perturbed parapositronium are analyzed in a wide range of the field. The feasibility of the application of the angular-correlation method to positron spin polarimetry is discussed.

Journal ArticleDOI
TL;DR: These measurements provide strong evidence that the positronium formation channel is having a significant absorption effect on the elastic-scattering channel, as is observed by the washing out of clear structure present in the measured elastic differential cross section below 9.0 eV.
Abstract: Relative differential cross sections for elastic scattering of 5--50-eV positrons by argon atoms at angles from 30{degree} to 134{degree} are measured in a crossed-beam experiment. These measurements provide strong evidence that the positronium formation channel (threshold=9.0 eV) is having a significant absorption'' effect on the elastic-scattering channel, as is observed by the washing out'' of clear structure (a maximum and minimum) present in the measured elastic differential cross section below 9.0 eV (where good agreement is found with theory) when the positron energy is increased to higher energies.

Journal ArticleDOI
TL;DR: In this article, complex coordinate rotation was used to investigate the P-wave resonances in positron-hydrogen scattering by using an Elaborate Hylleraas-type wavefunctions with up to N=1120 terms.
Abstract: P-wave resonances in positron-hydrogen scattering are investigated by using the method of complex-coordinate rotation. Elaborate Hylleraas-type wavefunctions with up to N=1120 terms are used in the present investigation. A resonance below the hydrogen N=2 threshold is found at E=-0.254 112 Ryd with a half width of 1/2 Gamma =8.1*10-6 Ryd. Three resonances below the positronium N=2 threshold are also located. The lowest member is determined to lie at E=-0.148 181 Ryd, with a half width of 1/2 Gamma =0.000 297 Ryd. Error estimates for the resonance parameters are also given.

Journal ArticleDOI
TL;DR: Positronium decay rates have been measured in silica gels of various pore sizes, at two temperatures, 297 and 77 K, and as a function of oxygen concentration as discussed by the authors.

Journal ArticleDOI
TL;DR: It is demonstrated that the mean-field theories of localization, which are derived from density functionals, can be obtained from the adiabatic model by employing a Gutzwiller type of approximation, and the application of the adovernment model, via the polymer representation, to the annihilation of positrons and positronium atoms in a fluid.
Abstract: This paper addresses two central issues in our understanding of self-trapping, or localization, of a light particle (e.g., electron, positron, or positronium atom) in a fluid. The first concerns the relationship between the mean-field calculations of localized states carried out from the mid-1950s until the present, and the direct calculation of light-particle (LP) properties from the adiabatic model. The second concerns the universality of the ring polymer representation of the LP, which has proved to be extremely useful for computing the properties of localized electrons. We first demonstrate that the mean-field theories of localization, which are derived from density functionals, can be obtained from the adiabatic model by employing a Gutzwiller type of approximation. We then study the application of the adiabatic model, via the polymer representation, to the annihilation of positrons and positronium atoms in a fluid. Expressions for the decay rate and its variance are derived in terms of the two- and three-point distribution functions for the polymer sites and positions of the atomic nuclei. The momentum distribution of the localized LP is shown to be proportional to the Fourier transform of the distribution function for the displacement between the ends of a broken polymer.

Journal ArticleDOI
TL;DR: In this paper, a new model of positronium (Ps) formation is proposed, where the first stage is the inelastic collisions of e+ through formation of an intermediate state (M*e+) by resonant energy absorption and e+ attachment.
Abstract: ‘‘Resonant model,’’ a new model of positronium (Ps) formation is proposed. The first stage of Ps formation is the inelastic collisions of e+ through formation of an intermediate state (M*.e+) by resonant energy absorption and e+ attachment. The second stage of Ps formation involves thermalized e+ and loosely bound electrons (excess electrons, excited molecules, and certain anions) through resonant electron transfer. Experimental results are reviewed from the viewpoint of the new model comparing it with the older models, the Ore model and the spur model.


Journal ArticleDOI
TL;DR: In this paper, the authors apply the first-order target continuum distorted wave (TCDW1) approximation to electron capture by positrons from helium into the ground state of positronium.
Abstract: The authors apply the first-order target continuum distorted wave (TCDW1) approximation to electron capture by positrons from helium into the ground state of positronium. Total cross sections for positronium formation are found to be in good agreement with the measurement of Fromme et al. (1986) above 160 eV. This is the energy range in which most theoretical results disagree with the measurements and indeed in previous classical trajectory Monte Carlo (CTMC) calculations it has been suggested that the experiments incorrectly describe large-angle ionization as charge transfer. The authors elaborate on this matter.

Journal ArticleDOI
TL;DR: In this paper, a five-state CCA was used to study positron-hydrogen scattering in the energy range k = 0.1 to 0.85 au, and the elastic d-wave phaseshifts were found to be in excellent agreement with those of existing reliable predictions.
Abstract: A five-state (H(1s, 2s, 2p, 3d), Ps(1s)) CCA has been employed to study positron-hydrogen scattering in the energy range k=0.1 to 0.85 au. The elastic d-wave phaseshifts are found to be in excellent agreement with those of existing reliable predictions. The effect of the positronium formation channel is found to be significant. The differential and total cross sections are reported by combining the present CCA results (l)2) and nearly exact numbers obtained using variational methods.

Journal ArticleDOI
TL;DR: Positron annihilation in liquid as well as in solid octadecane has been investigated by means of lifetime spectroscopy combined with magnetic quenching experiments as mentioned in this paper, which indicates that the longest lifetime component both in the liquid and solid phases must be ascribed to the decay of a positron-electron bound state.
Abstract: Positron annihilation in liquid as well as in solid octadecane has been investigated by means of lifetime spectroscopy combined with magnetic quenching experiments. An anomalous magnetic quenching in solid octadecane was detected, which cannot be explained in terms of the usual Zeeman effect on positronium with a modified electron contact density at the positron. A technique to extract the value of the relative contact density without making use of magnetic fields is used. The measurements indicate that the longest lifetime component both in the liquid and solid phases must be ascribed to the decay of a positron-electron bound state.

Journal ArticleDOI
TL;DR: The fully scaled theory of self-trapping is applied to the pickoff annihilation of orthopositronium and best agreement is obtained from a model that incorporates transitions between localized and extended states.
Abstract: The localization of a light particle (e.g., electron, positron, or positronium atom) in a fluid is known as self-trapping. In an earlier paper (B. N. Miller and T. L. Reese, Phys. Rev. A 39, 4735 (1989)) we showed that (1) the density-functional theories (DFT's) of self-trapping could be derived from a mesoscopic model that employs a quantum-mechanical description of the light particle and a classical description of the fluid, and (2) the application of scaling to the simplest variant of DFT results in a universal model for all fluids that obey the principle of corresponding states. In this paper we apply the fully scaled theory to the pickoff annihilation of orthopositronium. Predictions of three different versions of the theory are compared with the experimental measurements of McNutt and Sharma on ethane (J. Chem. Phys. 68, 130 (1978)) and Tuomisaari, Rytsola, and Hautojarvi on argon (Phys. Lett. 112A, 279 (1988)). Best agreement is obtained from a model that incorporates transitions between localized and extended states.

Journal ArticleDOI
TL;DR: In this article, a physically motivated approximation of the dominant non-adiabatic correction to the adiabatic potential at low scattering energies was proposed for positron-molecule collisions.
Abstract: A novel treatment of short-range correlation effects designed for use in calculating positron-atom and positron-molecule polarisation potentials is presented. The treatment is based on a physically motivated approximation that mimics the effect of the dominant non-adiabatic correction to the adiabatic potential at low scattering energies, namely virtual positronium formation. As an initial test of the method, positron-He scattering phaseshifts, scattering length and differential cross sections have been calculated and found to be in good agreement with accurate results. This method is immediately extendible to positron-molecule collisions and does not require significantly more work than a purely adiabatic treatment.


Journal ArticleDOI
TL;DR: Very accurate results for the elastic P-wave phaseshifts below the positronium formation threshold in positron-hydrogen atom collisions are obtained using only four terms in a discrete basis set expansion in the Schwinger variational principle as discussed by the authors.
Abstract: Very accurate results for the elastic P-wave phaseshifts below the positronium formation threshold in positron-hydrogen atom collisions are obtained using only four terms in a discrete basis set expansion in the Schwinger variational principle. Stationary results are also presented for higher partial waves up to l=6 for the scattering of electrons and positrons from the static potential in the incident momentum range 0.1-1.0 au.

Journal ArticleDOI
TL;DR: Positronium decay was used to characterize the pore structure of gel-silica monoliths as mentioned in this paper, and the results showed that the average pore radius of the silica gel monolith remains fairly constant during densification, while the total volume of pores and the total surface area of pore-solid interface decreases during the process.
Abstract: Positronium decay is used to characterize the pore structure of gel-silica monoliths. The results obtained from positronium decay are compared with results obtained by nitrogen gas adsorption for gel-silica monoliths of different pore sizes densified in the range of 180–1150°C. The results show that the average pore radius of the silica gel monoliths remains fairly constant during densification, while the total volume of pores and total surface area of the pore-solid interface decreases during the process. The positronium decay analysis reveals that the particle size remains constant during densification for gel-silica monoliths of small pore sizes, but changes for samples of larger pore diameter. It is shown that the average pore sizes obtained from the gas adsorption and positronium decay techniques yield comparable results.

Journal ArticleDOI
TL;DR: The local convergence and accuracy of wave functions obtained by direct solution of the Schroedinger equation with the help of the correlation-function hyperspherical-harmonic method are analyzed for ground and excited states of the helium atom and for the ground state of the positronium negative ion.
Abstract: The local convergence and accuracy of wave functions obtained by direct solution of the Schroedinger equation with the help of the correlation-function hyperspherical-harmonic method are analyzed for ground and excited states of the helium atom and for the ground state of the positronium negative ion. The inclusion of the cusp conditions into the correlation function is shown to be of crucial importance, not only near the coalescence points, but also away from them. The proper inclusion of all cusps yields for the ground state of the helium atom the local wave-function accuracy of about 10{sup {minus}7} for different interparticle distances. The omission of one of the cusps in the excited helium atom reduces the wave-function precision to 10{sup {minus}2} near the corresponding coalescence point and to 10{sup {minus}4}--10{sup {minus}5} away from it.

Journal ArticleDOI
TL;DR: The ionization of polyatomic molecules in their interaction with positrons is discussed with respect to the likely mechanisms expected at various positron kinetic energies and the rationale behind the study of these processes.