Showing papers on "Positronium published in 2000"

Positron Annihilation Lifetime Study of High and Low Free Volume Glassy Polymers: Effects of Free Volume Sizes on the Permeability and Permselectivity

Abstract: Free volume in a big group of glassy polymers distinguished by a wide range of gas permeability (P in the range 1−104 Barrer) was studied using the positron annihilation lifetime (PAL) method. To take into account an additional channel of decay of positronium (Ps), caused by the reaction with sorbed O2, measurements were performed both under ambient conditions and under a nitrogen atmosphere. The finite-term and continuous lifetime analyses (PATFIT and CONTIN programs) were used for the treatment of the positron annihilation decay curves. It was shown that better statistical fitting parameters can be obtained for four component PAL spectra of a vast majority of the glassy polymers studied. The presence of two Ps lifetimes (τ3 = 1.7−3.3 ns and τ4 = 2.7−10.9 ns) indicates that size distribution of free volume elements (FVE) in glassy polymers is typically bimodal. The presence of smaller and larger FVE (R3 = 2.5−3.5 A, R4 = 3.5−7.0 A) was proved by PATFIT analysis of the data. Continuous analysis clearly in...

Determination of pore-size distribution in low-dielectric thin films

Abstract: Positronium annihilation lifetime spectroscopy is used to determine the pore-size distribution in low-dielectric thin films of mesoporous methylsilsesquioxane. A physical model of positronium trapping and annihilating in isolated pores is presented. The systematic dependence of the deduced pore-size distribution on pore shape/dimensionality and sample temperature is predicted using a simple quantum mechanical calculation of positronium annihilation in a rectangular pore. A comparison with an electron microscope image is presented.

Positron/positronium annihilation as a probe for the chemical environment of free volume holes in polymers

Abstract: We assess the nature of the chemical environment of free volume “holes” in polymers of different chemical compositions using positronium annihilation spectroscopy. After the subtraction of a narrow parapositronium component obtained from a three-Gaussian fit from the conventional Doppler broadened annihilation energy line, the remaining broad distribution can be related to the nature and density of the chemical species in the vicinity of the holes. Element specific information on the annihilation site can be expressed in terms of an integral parameter, e.g., the normalized peak height. This method has an advantage in that it can be used as a fast and routine method.

Calculation of the lifetime of positronium in polymers via molecular dynamics simulations

Abstract: A method for the study of positronium annihilation in polymers is presented. The polymer configurations are generated by atomistic molecular dynamics simulations; their geometry is fully known, so that the size and the shape of the holes can be analyzed. The positronium lifetime in the free volume sites is calculated at finite temperature with the path integral Monte Carlo method in the framework of an extended model, which comprises the positronium–polymer interaction and the electron density in the polymer. The spectra and free volume properties are compared to results from positronium annihilation spectroscopy experiments. As an example, the method is applied to polystyrene at ambient conditions.

Order alpha(7)ln(1/alpha) contribution to positronium hyperfine splitting

Abstract: The logarithmically enhanced alpha(7)ln(1/alpha) contribution to the hyperfine splitting of the positronium ground-state energy levels is calculated in the framework of dimensionally regularized nonrelativistic quantum electrodynamics. The correction is negative and amounts to about 1/3 of the leading logarithmic alpha(7)ln (2)(1/alpha) one. The discrepancy between the experimental measurements and the theoretical prediction is reduced.

Photodetachment cross sections of the positronium negative ion

Abstract: The cross sections for the photodetachment from a weakly bound positronium negative ion Ps- below the threshold for the formation of Ps(n = 4) are calculated using the hyperspherical close-coupling method, and are compared with the corresponding spectra for the H- ion. Detailed resonance structures in the spectra near the Ps(n = 2, 3 and 4) thresholds are reported for the first time. The off-resonance cross section below the Ps(n = 2) threshold differs appreciably from that obtained by a variational calculation (Ward et al 1987 J. Phys. B: At. Mol. Phys. 20 127), but agrees well with the recent close-coupling calculations with a B-spline expansion (Igarashi et al 2000 Phys. Rev. A 61 032 710). The resonance energies and widths of the 1Po symmetry are generally in good agreement with the results of the complex-coordinate rotation calculation.

Positron and positronium chemistry by quantum Monte Carlo. V. The ground state potential energy curve of e+LiH

Abstract: The potential energy curve of e+LiH has been computed by means of diffusion Monte Carlo using explicitly correlated trial wave functions. This curve allows us to compute the adiabatic total and binding energies and the vibrational spectrum of e+LiH, and the adiabatic positron affinity of LiH. Using these results, we discuss the possibility to detect spectroscopically e+LiH in the gas phase, in order to have the first direct observation of a positron-containing system.

The effect of forward-angle scattering on positronium-gas total cross sections

Abstract: An investigation into forward-angle scattering of Ps atoms from atomic and molecular targets is reported. This effect has been evaluated by varying the angular acceptance of the detector, , in the range ±1.5° to ±6.4°. By assuming an average differential cross section over this range, values of the total cross section (extrapolated to 0) for Ps-He, Ar and H2 have been obtained. Where possible, the data are compared with available theoretical calculations.

Positron and positronium studies of irradiation-induced defects and microvoids in vitreous metamict silica

Abstract: To study irradiation-induced defects and structural microvoids in vitreous silica (v-SiO2), positron lifetime, angular correlation of positron annihilation radiation (ACAR), and electron spin resonance (ESR) were measured on v-SiO2 and quartz (c-SiO2) samples irradiated with fast neutrons up to a dose of 4.1×1020 n/cm2. Two kinds of positron-trapping defects have been found to form in v-SiO2 by fast neutron irradiation: type-I and type-II defects. Similar defects also appear in the irradiated c-SiO2, indicating that both the defects are common in v-SiO2 and c-SiO2. The detailed annealing and photo-illumination studies of positron annihilation and ESR for these two defects suggest that the type-I defects are non-bridging oxygen hole centers (NBOHC), while the type-II defects are oxygen molecules which cannot be detected by ESR. Higher dose irradiation than 1.0×1020 n/cm2 causes c-SiO2 to change to metamict (amorphous) phase (m-SiO2). Positronium (Ps) atoms are found to form in microvoids with an average radius of about 0.3 nm in the v-SiO2 and m-SiO2. This suggests that microvoids proved by Ps are structurally intrinsic open spaces and reflect the topologically disordered structure of these phases in the subnanometer scale.

T-matrix theory of positron annihilation on hydrogen

Abstract: The transition amplitude for electron-positron annihilation during positron-hydrogen scattering is formulated in terms of the half-shell T-matrix elements obtained from a solution of the momentum space Lippmann-Schwinger equation. The derived expression is well defined at energies below the positronium (Ps) formation threshold. However, singularities in the transition amplitude exist at energies above the Ps-formation threshold. The annihilation parameter for positron-hydrogen scattering is computed in a variety of models at energies below the Ps-formation threshold. Calculations with the largest basis reproduce previous variational calculations of ZJeff for the J = 0 and 1 partial waves. Accurate values of ZJeff were also given for the J = 2, 3 and 4 partial waves to give a summed Zeff that is probably the most accurate so far reported.

Excited states of the positronium molecule

Abstract: We investigate the spectrum of the positronium (Ps) molecule consisting of two electrons and two positrons by the stochastic variational method (SVM). By calculating the energies of states with Lπ=0±,1± and 2±, we confirm that only four states, 0+(A1), 0+(B2), 0+(E) and 1−(B2), are bound. We investigate the various properties of these bound states as well as the decay probabilities for the 2γ emission by electron–positron annihilation leaving a particular Ps final state. The binding energies of a biexciton, a Coulomb four-body system analogous to Ps2, are also examined as a function of the electron–hole mass ratio.

Total and positronium formation cross-sections in polyatomic molecules

Abstract: Total cross-section (TCS) studies of positron and electron impacts in polyatomic molecules are presented experimentally and theoretically. The polarization interaction in the positron and electron scattering processes by polyatomic molecules is discussed. Positronium (Ps) formation cross-sections in many polyatomic molecules are also measured at 2 eV above the corresponding Ps formation thresholds. And the results are analyzed based on the physical properties of the positron–molecule interaction systematically. Comparative studies of the vibrational excitation cross-sections by positron and electron impacts are also briefly described.

Positronium atom scattering by H 2 in a coupled-channel framework

Abstract: The scattering of ortho-positronium (Ps) by H2 has been investigated using a three-Ps-state (Ps(1s, 2s, 2p)H2 (X 1 g + )) coupled-channel model and using the Born approximation for higher excitations and ionization of Ps and B 1 u + and b 3 u + excitations of H2 . We employ a recently proposed time-reversal-symmetric non-local electron-exchange model potential. We present a calculational scheme for solving the body-frame fixed-nuclei coupled-channel scattering equations for Ps-H2 , which simplifies the numerical solution technique considerably. Ps ionization is found to have the leading contribution to target-elastic and all target-inelastic processes. The total cross sections at low and medium energies are in good agreement with experiment.

Supermultiplet structure of the doubly excited positronium negative ion

Abstract: Doubly excited intrashell resonance states of positronium negative ion, ${\mathrm{Ps}}^{\ensuremath{-}}$ associated with the $N=4$ and $N=5$ thresholds of Ps atom are calculated using the method of complex-coordinate rotation. Products of Slater-type orbitals are used to represent the two-electron wave functions. All possible intrashell states with angular momenta up to $L=6$ and $L=8$ for $N=4$ and $N=5$ thresholds, respectively, have been identified. Supermultiplet structures showing the rotational and vibrational character of the doubly excited resonance states of ${\mathrm{Ps}}^{\ensuremath{-}}$ are constructed.

O (α 3 ln α) corrections to positronium decay rates

Abstract: We compute O(alpha^3 ln alpha) corrections to the decay rates of para- and orthopositronium into two and three photons, respectively. For this calculation we employ the nonrelativistic QED regularized dimensionally and we explain how in this framework the logarithms of the fine structure constant can be extracted.

Intense positron beam at KEK

Abstract: A positron beam is a useful probe for investigating the electronic states in solids, especially concerning the surface states. The advantage of utilizing positron beams is in their simpler interactions with matter, owing to the absence of any exchange forces, in contrast to the case of low-energy electrons. However, such studies as low-energy positron diffraction, positron microscopy and positronium (Ps) spectroscopy, which require high intensity slow-positron beams, are very limited due to the poor intensity obtained from a conventional radioactive-isotope-based positron source. In conventional laboratories, the slow-positron intensity is restricted to 10 6 e + /s due to the strength of the available radioactive source. An accelerator based slow-positron source is a good candidate for increasing the slow-positron intensity. One of the results using a high intensity pulsed positron beam is presented as a study of the origins of a Ps emitted from SiO 2 . We also describe the two-dimensional angular correlation of annihilation radiation (2D-ACAR) measurement system with slow-positron beams and a positron microscope.

Convergence of two-centre expansions in positron-hydrogen collisions

Abstract: The question of convergence in two-centre expansions is addressed via the study of positron-hydrogen scattering within the s-wave model. Direct scattering, positronium formation, and breakup cross sections are found to converge if a sufficient number of states, centred on the hydrogen atom and positronium, are used to expand the total scattering wavefunction. Moreover, this occurs only if pseudostate expansions are applied to both the atomic and positronium centres.

Low-energy positronium-hydrogen elastic scattering using the six-state close coupling approximation

Abstract: Positronium-hydrogen elastic scattering is investigated using the six-state full close coupling approximation employing the basis set Ps(1s,2s,2p) + H(1s,2s,2p). S-wave singlet and triplet scattering and also the corresponding scattering length and effective range are compared with the corresponding predictions of Drachman and Houston and the Belfast group. We also report phase shifts up to l = 2 and the integrated elastic cross sections. Present results indicate that the effect of target excitation as well as the effect of excitations of both the atoms are significant in the energy region considered. We also report the angle integrated Ps(1s) + H(1s) → Ps(2p) + H(1s) and Ps(1s) + H(1s) → Ps(1s) + H(2p) excitation cross sections.

Positron scattering from argon and krypton

Abstract: The elastic cross-sections for positron scattering from Ar and Kr have been determined by the polarized-orbital method while the excitation cross-sections have been calculated within a distorted-wave framework in an energy range up to 200 eV. The excitation cross-sections are compared with available experimental results for argon. We analyze the relative contributions to the total cross-section of our elastic and excitation cross-sections and of the cross-sections, calculated by other authors, for positronium formation and for ionization. We compare these results with experimental measurements for the total cross-section. Our results are in satisfactory agreement with the experimental measurements except at low energies, where positronium formation is important.

A two-chain path integral model of positronium

Abstract: We have used a path integral Monte Carlo technique to simulate positronium (Ps) in a cavity. The primitive propagator is used, with a pair of interacting chains representing the positron and electron. We calculate the energy and radial distribution function for Ps enclosed in a hard, spherical cavity, and the polarizability of the model Ps in the presence of an electrostatic field. We find that the positron distribution near the hard wall differs significantly from that for a single particle in a hard cavity. This leads to systematic deviations from predictions of free-volume models which treat Ps as an effective, single particle. A virial-type estimator is used to calculate the kinetic energy of the particle in the presence of hard walls. This estimator is found to be superior to a kinetic-type estimator given the interaction potentials, cavity sizes, and chain lengths considered in the current study.

Extracting generalized multiphoton ionization cross-sections from nonperturbative time-dependent calculations: An application in positronium

Abstract: Ab initio time-dependent (TD) calculations of the behavior of positronium (Ps) under strong subpicosecond laser pulses are presented. The results are compared with results in H through scaling. It is found that a substantial amount of the population can be found in excited states after the pulse. In the perturbative regime, generalized multiphoton ionization cross-sections are extracted from the results of the time-dependent calculations. The generalized cross-sections are used to predict the response of Ps to nanosecond laser pulses at wavelengths of current experimental interest. Beyond the application to Ps, the generality of the method for extracting generalized cross-sections from TD nonperturbative calculations is discussed.