Showing papers on "Positronium published in 2012"
TL;DR: In this paper, the authors demonstrate the production of Rydberg positronium (Ps) atoms in a two-step process, comprising incoherent laser excitation, first to the 2P3 state and then to states with principal quantum numbers ranging from 10 to 25.
Abstract: We demonstrate experimentally the production of Rydberg positronium (Ps) atoms in a two-step process, comprising incoherent laser excitation, first to the 2P3 state and then to states with principal quantum numbers ranging from 10 to 25. We find that excitation of 2P3 atoms to Rydberg levels occurs very efficiently (∼90%) and that the ∼25% overall efficiency of the production of Rydberg atoms is determined almost entirely by the spectral overlap of the primary excitation laser and the Doppler broadened width of the 1S3-2P3 transition. The observed efficiency of Rydberg Ps production can be explained if stimulated emission back to the 2P states is suppressed, for example, by intermixing of the Rydberg state Stark sublevels. The efficient production of long-lived Rydberg Ps in a high magnetic field may make it possible to perform direct measurements of the gravitational free fall of Ps. © 2012 American Physical Society.
70 citations
TL;DR: Ps(2) molecules created in a porous silica film and also in vacuum from an Al(111) crystal, were resonantly excited and then photoionized by pulsed lasers, providing conclusive evidence for the production of this molecular matter-antimatter system and its excited state.
Abstract: We report optical spectroscopic measurements of molecular positronium (Ps(2)), performed via a previously unobserved L=1 excited state. Ps(2) molecules created in a porous silica film, and also in vacuum from an Al(111) crystal, were resonantly excited and then photoionized by pulsed lasers, providing conclusive evidence for the production of this molecular matter-antimatter system and its excited state. Future experiments making use of the photoionized vacuum L=1 Ps(2) could provide a source of Ps(+) ions, as well as other multipositronic systems, such as Ps(2)H(-) or Ps(2)O.
58 citations
TL;DR: Improvements to the methodology that could lead to the ∼ppm level of precision required to address the long-standing discrepancy between QED calculations and precision experiments using microwave radiation to induce transitions between Zeeman shifted triplet Ps states are described.
Abstract: We report Doppler-free measurements of the positronium (Ps) Lyman-α transition using saturated absorption spectroscopy. In addition to a Lamb dip at wavelength λ(L) = 243.0218 ± 0.0005 nm, we also observed a crossover resonance at λ(C) = 243.0035 ± 0.0005 nm, arising from the excitation of 1(3)S(1) atoms to Zeeman mixed 2P states, followed by stimulated emission to the 1(1)S(0) ground state. Since (λ(L)-λ(C)) is related to the Ps hyperfine interval E(hfs), this observation constitutes the first optical measurement of this quantity and yields E(hfs) = 198.4 ± 4.2 GHz. We describe improvements to the methodology that could lead to the ∼ppm level of precision required to address the long-standing discrepancy between QED calculations and precision experiments using microwave radiation to induce transitions between Zeeman shifted triplet Ps states.
44 citations
TL;DR: A novel full-differential Monte Carlo event-by-event simulation suitable for electron and positron transport in gaseous and liquid water is described, which should represent an accurate tool for dose calculation at the nanometric scale, by providing a detailed spatial distribution of energy deposits.
Abstract: Purpose: We describe here a novel full-differential Monte Carlo (MC) event-by-event simulation, for modelling electron and positron histories in liquid and gaseous water, with impact energies ranging from the water excitation threshold (7.4 eV) to 10 keV. This new track-structure code is named EPOTRAN, an acronym for Electron and POsitron TRANsport in water. Material and methods: All the processes induced by both electrons and positrons are studied in detail via theoretical differential and total cross sections, calculated within the quantum mechanical framework by using the partial-wave method. Elastic and inelastic interactions are then successively reviewed, including in particular an original description of the positron-induced capture process leading to Positronium formation. Results: Total and differential cross sections are reported and compared with a large set of existing measurements. Rather good agreement is generally observed over the considered energy range. Conclusions: This work rep...
38 citations
TL;DR: In this article, mesoporous silica films with different pore morphologies and its emission from the surfaces were studied using the ortho-positronium time-of-flight method and positron annihilation.
Abstract: Diffusion of ortho-positronium ($o$-Ps) in mesoporous silica films with different pore morphologies and its emission from the surfaces were studied using the Ps time-of-flight method and positron annihilation $\ensuremath{\gamma}$-ray energy spectroscopy. Probabilities of $o$-Ps formation are almost the same in the studied films, however, the emission intensity of $o$-Ps from the film with much larger cagelike pores is significantly less than that from the film with much smaller tubelike pores, where Ps diffusion between cages is feasible by tunneling. A simple strategy is suggested to use mesoporous silica film for the generation of dense enough Ps for many-positronium experiments.
35 citations
TL;DR: In this article, the convergent close-coupling formulation for positron scattering from noble gases (Ne, Ar, Kr and Xe) within the single-center approximation is presented.
Abstract: We present the convergent close-coupling formulation for positron scattering from noble gases (Ne, Ar, Kr and Xe) within the single-center approximation. Target functions are described in a model of six p-electrons above an inert Hartree-Fock core with only one-electron excitations from the outer p 6 shell allowed. Target states have been obtained using a Sturmian (Laguerre) basis in order to model coupling to ionization and positronium (Ps) formation channels. Such an approach is unable to yield explicit Ps-formation cross sections, but is valid below this threshold and above the ionization threshold. The present calculations are found to show good agreement with recent measurements.
31 citations
TL;DR: In this article, the effect of the screened Coulomb potential on positronium formation process was investigated by using the Debye-Huckel potential, and the present results are compared with available theoretical calculations.
Abstract: Positronium (Ps) formation cross sections (n = 1, 2) in positron-hydrogen collisions in Debye plasma environment are calculated using the screening approximation model for various Debye screening lengths from the Ps formation thresholds to 50 eV. The effect of the screened Coulomb potential on Ps formation process is investigated by using the Debye-Huckel potential. The present results are compared with available theoretical calculations.
27 citations
TL;DR: In this article, a review of the physics of fast positronium formation in condensed media is presented, including the Ore model, quasifree Ps state, and the PS bubble model.
Abstract: This chapter reviews the following items: 1. Energy deposition and track structure of fast positrons: ionization slowing down, number of ion-electron pairs, typical sizes, thermalization, electrostatic interaction between e+ and its blob, effect of local heating; 2. Positronium formation in condensed media: the Ore model, quasifree Ps state, intratrack mechanism of Ps formation; 3. Fast intratrack diffusion-controlled reactions: Ps oxidation and ortho-para conversion by radiolytic products, reaction rate constants, interpretation of the PAL spectra in water at different temperatures; 4. Ps bubble models. "Non-point" positronium: wave function, energy contributions, relationship between the pick-off annihilation rate and the bubble radius.
26 citations
TL;DR: In this article, an energy-tunable mono-energetic positronium beam was produced using photodetachment of the POS negative ion, a bound state of one positron and two electrons, by bombarding a Na-coated tungsten surface with a pulsed slow positron beam and accelerated using a static electric field.
Abstract: We report here the production of an energy-tunable monoenergetic positronium beam using photodetachment of the positronium negative ion, a bound state of one positron and two electrons. The ions were produced efficiently by bombarding a Na-coated tungsten surface with a pulsed slow positron beam and accelerated using a static electric field. On irradiating the ions with a high intensity pulsed laser light, a positronium beam with a hitherto unrealized energy range of up to 1.9 keV has been produced in an ultra high vacuum environment.
25 citations
TL;DR: In this article, the Coulomb screening was applied to positronium formation in positron-helium collisions under the effect of a Coulomb screen, e+ + He(1s 2) → Ps(100) + He+(100), using the second-order distorted-wave approximation (DWA) and taking into consideration the screened dipole polarisation potential at low and intermediate incident positron energies in the range, 6 − 500 eV.
Abstract: A study is conducted on positronium (Ps) formation in positron-helium collisions under the effect of a Coulomb screening, e+ + He(1s
2) → Ps(100) + He+(100), using the second-order distorted-wave approximation (DWA) and taking into consideration the screened dipole polarisation potential at low and intermediate incident positron energies in the range, 6 − 500 eV. The theoretical model for the scattering calculation is fairly accurate and predicts a number of interesting features in the total and differential cross sections for the screening parameter, μ = 0.0−0.4. The existence of resonances in the S-, P-, D- and F-partial waves has been reported in this work. Surface plots of the total and differential cross sections have also been presented to illustrate the interesting nature of the Ps formation in this scattering process.
25 citations
TL;DR: In this paper, the Debye-Huckel potentials have been used to model the interactions among the charged particles in a weakly coupled plasma to obtain distorted-wave scattering amplitudes.
Abstract: Dynamics of positronium (Ps) formation in an arbitrary s-state from an arbitrary s-state of the hydrogen atom in weakly coupled plasma has been investigated within the framework of a distorted-wave theory. The interactions among the charged particles in the plasma have been represented by Debye-Huckel potentials. Using simple variational hydrogenic wave functions and arbitrary order derivatives of the general three-denominator Lewis integral partial-wave scattering amplitudes have been obtained in closed forms. These distorted-wave scattering amplitudes have been used to make a detailed study on differential and total cross sections. It has been found that screening of the interaction potentials has significant effect on the scattering dynamics. Some notable features of cross sections have also been revealed.
TL;DR: In this article, the authors measured the Ortho -positronium ( o -Ps) lifetime in alkanes and alcohols in a broad range of temperatures and the results were compared with calculations in the framework of a standard bubble model.
TL;DR: In this article, the two-center convergent-close-coupling method is extended to the calculation of positron scattering from atoms with two valence electrons above an inert ion core.
Abstract: The two-center convergent-close-coupling method is extended to the calculation of positron scattering from atoms with two valence electrons above an inert ion core. Calculations of positron scattering on magnesium have been performed over a wide energy range. Elastic scattering, Positronium formation, target excitation, and direct ionization cross sections have been calculated. Results are in reasonably good agreement with the experimental data where available. Current results confirm the existence of a low-energy shape resonance predicted by variational and single-center convergent-close-coupling methods.
TL;DR: In this article, the authors describe the evolution of positronium from its injection into a liquid until its annihilation, and discuss several stages of the evolution: energy deposition and track structure of fast positrons: ionization slowing down, number of ion-electron pairs, typical sizes, thermalization, electrostatic interaction between and the constituents of its blob, and effect of local heating.
Abstract: The present approach describes the
TL;DR: The results suggest that free volume sites can expand towards an equilibrium size, determined as the equilibrium Ps-bubble size defined earlier for low-molecular-weight liquids.
Abstract: The possibility of positronium induced free volume cavity expansion in some polymers above the glass transition temperature was investigated using experimental positron annihilation lifetime data from the literature for polydimethylsiloxane, polyisobutylene, and polybutadiene as function of temperature. The results suggest that free volume sites can expand towards an equilibrium size, determined as the equilibrium Ps-bubble size defined earlier for low-molecular-weight liquids. The expansion can be explained by the increase of molecular mobility and hence decrease of relaxation times, which at the higher temperatures approach the o-Ps lifetimes. Nanoscale viscosities were estimated using Navier-Stokes equation and were found to be several orders of magnitude lower than the macroscopic viscosity at the same temperature.
TL;DR: In this article, the branching ratio of orthopositronium decay into an invisible mode is calculated in the context of mirror world models, where ordinary positronium can disappear from our world due to oscillation into its mirror twin.
Abstract: We present a calculation of the branching ratio of orthopositronium decay into an invisible mode, which is done in the context of mirror world models, where ordinary positronium can disappear from our world due to oscillation into its mirror twin. In this revision we clarify some formulas and approximations used previously, correct them at some places, add new effects relevant for a feasible experiment, and finally perform a combined analysis. We include into consideration various effects due to external magnetic and electric fields, collisions with cavity walls, and scattering off gas atoms in the cavity. Oscillations of the Rydberg positroniums are also considered. To perform a numerical estimate in a realistic case we wrote computer code, which can be adopted in any experimental setup. Its work is illustrated with an example of a planned positronium experiment within the AEgIS project.
TL;DR: In this paper, the transition from ortho-positronium to para-positionionium has been observed with 5 σ CL, which is the first observation of M1 transition in (sub)Terahertz region.
Abstract: Positronium is an ideal system for research on QED, especially in a bound state. A discrepancy (3.9σ) is found recently between measured HFS values and the QED prediction (including up-to O(α3 log α − 1), where α is the fine-structure constant.). It might be due to a contribution of unknown new physics or common systematic problems in all the previous measurements. A new method to measure HFS directly is performed using a high power gyrotron. The transition from ortho-positronium to para-positronium has been observed with 5 σ CL, which is the first observation of M1 transition in (sub)Terahertz region. New technologies of high power gyrotrons are developed for precision spectroscopy.
TL;DR: A strong linear correlation between the (5)D(4) Tb(iii) energy level excited state lifetime and the positronium formation probability has been observed, indicating that the ligand-to-metal charge transfer LMCT states act in both luminescence quenching and positronia formation inhibition, as previously proposed.
Abstract: Positronium formation in the bimary molecular solid solutions Tb1−xEux (dpm)3 (dpm = dipivaloylmethanate) has been investigated. A strong linear correlation between the 5D4 Tb(III) energy level excited state lifetime and the positronium formation probability has been observed. This correlation indicates that the ligand-to-metal charge transfer LMCT states act in both luminescence quenching and positronium formation inhibition, as previously proposed. A kinetic mechanism is proposed to explain this correlation and shows that excited electronic states have a very important role in the positronium formation mechanism.
Journal Article•
TL;DR: The efficient production of long-lived Rydberg Ps in a high magnetic field may make it possible to perform direct measurements of the gravitational free fall of Ps, if stimulated emission back to the 2P states is suppressed, for example, by intermixing of the Ryd Berg state Stark sublevels.
DOI•
01 Jun 2012TL;DR: In this article, a combined experimental and theoretical overview of positronium collisions with atoms and molecules is presented, including the Born approximation, coupled-state methods, variational techniques, stabilization methods, model potentials, Monte Carlo methods, impulse approximation, and resonances.
Abstract: This article presents a combined experimental and theoretical overview of positronium collisions with atoms and molecules. It starts with a description of the basic properties of positronium. This is followed by a survey of experimental methods. These include positron annihilation lifetime spectroscopy (PALS and SSPALS), angular correlation of annihilation radiation (ACAR), Doppler broadening (DB), positronium beam experiments, and many-positron techniques. Theoretical developments are next described, including the Born approximation, coupled-state methods, variational techniques, stabilization methods, model potentials, Monte Carlo methods, the impulse approximation, and resonances. There then follows a critical comparison between experiment and theory on low-energy momentum transfer cross-sections, total cross-sections, integral and differential ionization cross-sections, and positronium-positronium scattering. Comparison is also made with equivelocity free electron and positron scattering, highlighting the remarkable result that positronium scattering tends to follow that of a bare electron. The article concludes with a look towards future directions.
TL;DR: A large amount of work has been performed on positron and positronium (Ps) binding to atoms as mentioned in this paper, and the known binding energies were fit to a simple expression involving a few common physical properties and then used it to predict the Ps-binding energies of the other atoms.
Abstract: A large amount of work has been performed on positron and positronium (Ps) binding to atoms. Positron binding was treated in our last paper [Phys. Rev. A 83, 032504 (2011)], of which the present paper is a continuation. For Ps binding, 12 atoms have been found to bind with Ps, and 2 atoms have been found not to bind with Ps. We fit the known binding energies to a simple expression involving a few common physical properties and then use it to predict the Ps-binding energies of the other atoms. The model and results appear to be reliable.
TL;DR: Strong evidences that the electronic delocalization does not occur at both temperatures are found and suggest that the absence of positronium in Eu(III) complexes cannot be explained by a ligand/metal charge transfer process.
TL;DR: In this paper, the effect of screened Coulomb potentials on the high-lying doubly excited resonance states of the positronium negative ion in the framework of complex-scaling method was investigated.
Abstract: We have investigated the effect of screened Coulomb potentials on the high-lying doubly excited resonance states of the positronium negative ion in the framework of complex-scaling method. Highly correlated wave functions in Hylleraas coordinates are used. The resonance parameters below the Ps (2s2S) and Ps (3s2S) thresholds, for various screening parameters, are reported.
TL;DR: In this paper, the creation of directed fluxes of positronium atoms using the optical dipole force from short, pulsed travelling optical lattices has been studied and it has been found that accelerations in excess of 1014g are possible.
Abstract: The creation of directed fluxes of positronium atoms using the optical dipole force from short, pulsed travelling optical lattices has been studied. By exploiting the favourable polarizability-to-mass ratio of positronium it has been found that accelerations in excess of 1014g are possible. Simulations have been performed for several cases in which lattice beams capture a substantial fraction of a cloud of ground state ortho-positronium atoms, assumed to be characterised by an initial temperature of 300?K. We show that, using conventional laser sources, bunches of positronium atoms can be accelerated, well within the ortho-positronium vacuum lifetime of 142?ns, to a wide variety of kinetic energies in the eV to 100s of eV range and with well-defined energy spreads. The final kinetic energy of the positronium can be tuned using the laser intensity, the pulse duration or the rate of change of the frequency between the two beams which form the lattice.
TL;DR: In this paper, a Gaussians basis for the many-body system consisting of a He nucleus, three electrons, and a positron was studied using an explicitly correlated Gaussian basis and a stochastic variational method for the optimization of the basis.
Abstract: (Received 29 February 2012; published 16 May 2012)The many-body system comprising a He nucleus, three electrons, and a positron has been studied using anexplicitly correlated Gaussians basis and a stochastic variational method for the optimization of the basis. Thepurpose has been to clarify to which extent the system can be considered as a distinguishable positronium (Ps)atom interacting with a He atom and, thereby, to pave the way to a practical atomistic modeling of Ps states andannihilation in matter. The maximum value of the distance between the positron and the nucleus is constrainedand the Ps atom at different distances from the nucleus is identified from the electron and positron densities,as well as from the electron-positron distance and center-of-mass distributions. The polarization of the Ps atomincreasesasitsdistancefromthenucleusdecreases.ThecontactdensityoftheelectronsofHewiththepositronisdepleted, particularly when the overlap is small. The ortho-Ps pick-off annihilation rate calculated as the overlapof the positron and the free He electron densities has to be corrected for the observed depletion, especially atlarge pores or voids.DOI: 10.1103/PhysRevA.85.052707 PACS number(s): 34 .80.Lx, 31.15.ac, 36.10.Dr
TL;DR: In this paper, the effects of channel-coupling scheme, target continuum and positronium (Ps) formation channels on the resonance energy and resonance width are discussed and the Wigner cusp structures at the inelastic channel-opening thresholds in positron-sodium scattering cross sections are also found.
Abstract: S-, P- and D-wave Feshbach resonances in positron-sodium scattering have been investigated by using the momentum space coupled-channels optical method. The target continuum and positronium (Ps) formation channels are included via an optical potential. Feshbach resonances below the target excitation and Ps (n = 2) formation thresholds are predicted and the effects of channel-coupling scheme, target continuum and Ps formation channels on the resonance energy and resonance width are discussed. We have also found the Wigner cusp structures at the inelastic channel-opening thresholds in positron-sodium scattering cross sections.
TL;DR: In this paper, depth dependent DB measurements on thin metallic films using the high intense monoener-getic positron beam NEPOMUC were presented, and the positron diffusion length in thin films was determined by the Doppler broadening of the annihilation radiation and additionally the formation of positronium (Ps) at the surface.
TL;DR: Positron chemistry refers to chemical processes of high-energy positrons injected into molecular substances, the most interesting of which is the formation of positronium (Ps), the hydrogen-like bound state between a positron and an electron as discussed by the authors.
Abstract: Positron chemistry refers to chemical processes of high-energy positrons injected into molecular substances, the most interesting of which is the formation of positronium (Ps), the hydrogen-like bound state between a positron and an electron. Ps is formed predominantly by fast intra-track radiation chemical processes. In polymers it tends to be localized in intra/inter-molecular open space in the sparsely packed amorphous structure. Whilst short-lived singlet para-positronium (p-Ps) undergoes self-annihilation, the positron in long-lived triplet ortho-positronium (o-Ps) annihilates with one of the spin opposite electrons bound in the surrounding polymer molecules. This process is called pick-off annihilation. The pick-off annihilation lifetime reflects the polymer chain packing through the size of the volume, where Ps is localized. Positrons are used to probe the amorphous structure of various polymeric systems. In this article, basic concepts and experimental techniques of positron chemistry in polymers as well as applications to the characterization of functional polymeric materials are overviewed.
TL;DR: In this article, the authors present experimental and theoretical cross sections for positron collisions with ethene molecules, for energies from 0.1 eV up to 70 eV.
Abstract: We present experimental and theoretical cross sections for positron collisions with ethene molecules. The experimental total cross sections (TCSs) were obtained with a linear transmission technique, for energies from 0.1 eV up to 70 eV. The calculations employed the Schwinger multichannel method and were performed in the static plus polarization approximation for energies up to 10 eV. Our calculated elastic cross sections indicate a Ramsauer-Townsend minimum around 2.8 eV and a virtual state, in agreement with previous calculations by da Silva et al. [Phys. Rev. Lett. 77, 1028 (1996)]. We found reasonable agreement between the calculated elastic integral cross section and the measured total cross section below the positronium formation threshold. The present results are also in quite good agreement with available theoretical and experimental data, although for the experiments this is only true for TCSs above about 7 eV.