Showing papers on "Positronium published in 1999"

Positronium annihilation in mesoporous thin films

Abstract: Depth-profiled positronium lifetime spectroscopy is used to probe the pore characteristics (size, distribution, and interconnectivity) in porous, low-dielectric silica films. The technique is sensitive to the entire void volume, both interconnected and isolated, even if the film is buried beneath a metal or oxide layer. Our extension of a simple quantum mechanical model of Ps annihilation in a pore adequately accounts for the temperature and pore size dependence of the Ps lifetime for pore sizes in the range from 0.1 nm to 600 nm. It is applicable to any porous media.

Extension of the equation for the annihilation lifetime of ortho-positronium at a cavity larger than 1 nm in radius

Abstract: Evaluations of cavity concentrations and their size distributions in industrially important materials by positron annihilation lifetime methods require a well calibrated ortho-positronium (o-Ps) lifetime standard curve as a function of pore size. A semiempirical equation with a fitting parameter obtained by Nakanishi and Jean has been successfully utilized in various porous and polymeric materials to estimate pore sizes up to approximately 1 nm in radius. However, as experimental data accumulate recently, it has become clear that the equation no longer yields a good correlation between o-Ps lifetimes and the pore size for the pore radius larger than 1 or o-Ps lifetime longer than ∼20 ns. Therefore, we assumed for the larger pore that o-Ps behaves more like a quantum particle, bouncing back and forth between the energy barriers as the potential well becomes large. An equation derived from this hypothesis gives excellent fitting with experimental data despite a crude approximation used in our model. More im...

Positronium S state spectrum: analytic results at O(m alpha^6)

Abstract: We present an analytic calculation of the O(m{alpha}{sup 6}) recoil and radiative recoil corrections to energy levels of positronium nS states and their hyperfine splitting. A complete analytic formula valid to O(m{alpha}{sup 6}) is given for the spectrum of {ital S} states. Technical aspects of the calculation are discussed in detail. Theoretical predictions are given for various energy intervals and compared with experimental results. {copyright} {ital 1999} {ital The American Physical Society}

Positronium scattering by helium

Abstract: Coupled-state calculations are presented for $\mathrm{Ps}(1s)+\mathrm{He}{(1}^{1}S)$ scattering in the energy range 0--40 eV. Up to 22 Ps states are employed but only one He state is retained. Pseudostates are used to represent the Ps continuum. First Born estimates of cross sections in which the He is excited or ionized are found to be negligible in the chosen energy range. Calculation shows that ionization of the Ps is the main process at the higher energies. Comparison is made with available experimental measurements of the total cross section and with other theories. At low energies there is not only serious conflict between different experiments but also between the different theoretical approximations. In particular, the calculations of Biswas and Adhikari [Phys. Rev. A 59, 363 (1999)] and Peach (unpublished) predict a much smaller elastic cross section at low energies than that obtained in the present work. The relative merits of the different approximations are discussed.

Bound-state properties of the positronium negative ion Ps -

Abstract: Various geometrical and energetical ~bound-state! properties of the positronium negative ion (Ps or eee) are determined by using highly accurate variational wave functions. These wave functions have been constructed by applying the advanced two-stage strategy proposed by Frolov @Phys. Rev. A 57, 2436 ~1998!#. The determined total energy of the ground state E520.262 005 070 232 975 7 a.u. is the lowest and most accurate value obtained for this system to date ~the corresponding binding energy equals 20.326 674 721 317 821 eV). The computation of the second-order relativistic corrections (.a) to the total energy is discussed also. The general form of the Breit-Pauli Hamiltonian in the relative coordinates for the Ps ion is presented. @S1050-2947~99!04910-0#

Elastic scattering and positronium formation in low-energy positron-helium collisions

Abstract: Ab initio variational calculations are made of the elastic scattering and positronium formation cross sections for positron-helium scattering below the first excitation threshold of the target. Three variants of the Kohn variational method are used with very flexible trial wavefunctions and a very accurate helium wavefunction. The elastic scattering and positronium formation cross sections are believed to have converged to within 5% and 10%, respectively. The calculated positronium formation cross section displays a similar energy dependence to that of recent experimental results but with a 25% difference in magnitude. However, the theoretical total cross section, both above and below the positronium formation threshold, agrees with the experimental measurements to within 10%. Threshold effects in the s-wave cross sections manifest themselves as a significant feature in the total cross section. Similarities with the corresponding results for positron-hydrogen scattering are discussed.

Lagrange-mesh calculations of three-body atoms and molecules

Abstract: The Lagrange-mesh numerical method has the simplicity of a mesh calculation and the accuracy of a variational calculation. A three-dimensional Lagrange-mesh method based on zeros of Laguerre polynomials is applied to the study of the ground states of the helium atom, the hydrogen and positronium negative ions and the hydrogen molecular ion. The calculations lead to big sparse matrices. Highly accurate energies are obtained, which essentially agree with recently published results or improve them. The accuracy is estimated by varying the mesh size and two nonlinear scale parameters. Different mean values of observables are obtained very simply with the Gauss quadrature approximation associated with the mesh.

Positron binding to a model alkali atom

Abstract: The fixed core stochastic variational method is used to investigate positron binding to a model alkali atom with a continuously adjustable ionization potential. Positron binding is possible for model atoms which have ionization potential (IP) ranging from 0.1767 to 0.479 Hartree (corresponding to dipole polarizabilities ranging from 209 to 23.5 a03). Results of the model indicate that positron binding is likely for gold, but not for potassium, rubidium or caesium. The annihilation rate was largest (1.97 × 109 s-1) when the IP is smallest and smallest (0.07 × 109 s-1) when the IP is largest. The presence of a positronium (Ps) cluster configuration in the wavefunction is found to be important for an accurate estimate of the annihilation rate even in circumstances when the positron and electron are located large distances apart.

The effect of visible light irradiation on positronium formation in polyethylene at low temperature

Abstract: b Department of Materials Science, Japan Atomic Energy Research Institute Tokai, Naka, Ibaraki 319-1106, Japan The Graduate University for Advanced Studies High Energy Accelerator Research Organization (KEK) 1-1, Oho, Tsukuba, Ibaraki 305-0801, Japan dHigh Energy Accelerator Research Organization (KEK) 1-1, Oho, Tsukuba, Ibaraki 305-0801, Japan Research Center for Nuclear Science and Technology, University of Tokyo Tokai, Naka, Ibaraki 319-1106, Japan

Scaling of hydrogenic atoms and ions interacting with laser fields: Positronium in a laser field

Abstract: Scaling laws are derived for hydrogenlike atoms and ions interacting with laser fields. In particular, the scaling of (appearance) intensities is derived. This scaling is independent of the physical mechanism responsible for the ionization process, be it tunneling or multiphoton ionization. It provides a firm basis and explanation for the validity of earlier models for the estimation of appearance intensities and of the extrapolation into the tunneling regime of scaling laws obtained through lowest-order perturbation theory. As an example of the applicability of the scaling laws, we calculate two-, three-, and six-photon generalized cross sections and ionization rates for positronium at laser frequencies of current experimental interest.

O(m alpha^7 ln(alpha)^2) corrections to positronium energy levels

Abstract: We present a calculation of the O(m alpha^7 ln(alpha)^2) corrections to positronium energy levels. The result is used to estimate the current uncertainty in theoretical predictions of the positronium spectrum.

Positronium Annihilation Lifetime and Dynamic Mechanical Studies of γ-Relaxation in BPA-PC and TMBPA-PC Plasticized by TOP

Abstract: Positronium annihilation lifetime spectroscopy (PALS) was employed to study the structure and dynamic properties of glassy BPA-PC and TMBPA-PC containing tris(2-ethylhexyl) phosphate (TOP) as diluent molecules. For BPA-PC/TOP blends, the results showed that the total hole volume fraction is insensitive to the addition of TOP up to 10 wt % but increases at higher TOP concentration. However, in the case of TMBPA-PC/TOP blends, a different trend was observed. The total hole volume fraction decreases monotonically with TOP concentration up to 15% and thereafter increases slightly from 15 to 20%. Dynamic mechanical relaxation was also used to monitor the dynamics of the blends. This part of the work was motivated by a prediction of the coupling model (made before the experimental measurement was carried out) that addition of TOP should have opposite effects on the γ-relaxation times of BPA-PC and TMBPA-PC. Indeed, it was found by dynamic mechanical measurement at constant frequency that, while the γ-relaxation...

Ionization of argon and krypton by positron impact

Abstract: Total single and double ionization cross sections for argon and krypton by positron impact have been measured for projectile energies ranging from threshold to ~1 keV. The single ionization cross sections are seen to behave as expected, showing large contributions from positronium formation at low energies. In the double ionization cross sections there seems to be a lack of positronium formation at threshold, while a clear contribution from the process is seen at intermediate energies.

Computational investigation of positron scattering from C 60

Abstract: The low-energy (0\char21{}6-eV) behavior of positrons scattering from gaseous ${\mathrm{C}}_{60}$ molecules is examined through a computational study which employs a nonempirical modeling of all the relevant interaction forces (static, correlation, and polarization potentials). There is uncertainty concerning the most appropriate correlation-polarization potential to use in the study of positron-molecule scattering. Here we have considered two such potentials which are representative of the potentials which have been proposed for this purpose. The coupled quantum scattering equations are solved in the body-fixed frame of reference, and the fixed-nuclei approximation is used to decouple the nuclear motion during the collision. The elastic integral cross sections without positronium formation exhibit strong one-particle resonances in the very-low-energy region up to $\ensuremath{\sim}4$ eV. The low-angular-momentum scattering components are shown to be the dominant contributions to trapped state wave functions. The existence of resonances in positron scattering from molecules is unusual. In the system studied here there are two types of resonances. First there are angular-momentum barrier resonances with the positron trapped outside of the ${\mathrm{C}}_{60}$ cage. Second, there are resonances of low angular momentum $(ll~2)$ with the positron trapped inside of the ${\mathrm{C}}_{60}$ cage by the repulsive interaction between the positron and the nuclei. The possibility of the experimental observation of these resonances is also discussed.

High-angular-momentum ( L >~ 3 ) doubly excited resonance states of the positronium negative ion

Abstract: We present a complex-coordinate rotation calculation of the doubly excited high-angular-momentum $(Lg~3)$ resonance states of the positronium negative ion, ${\mathrm{Ps}}^{\ensuremath{-}},$ associated with the $N=3,$ $N=4$, and $N=5$ thresholds of the Ps atom. Products of Slater-type orbitals are used to represent the two-electron wave functions. For the Ps $N=5$ threshold, states with angular momentum up to $L=8$ are calculated. Results for doubly excited intrashell states are used to construct the supermultiplet structures of the energy levels of ${\mathrm{Ps}}^{\ensuremath{-}},$ revealing rotational and vibrational series similar to those for an XYX triatomic molecule.

Quantitative determination of a sodium dodecyl sulfate micellar radius through positron annihilation lifetime spectroscopy

Abstract: Rigorous equations are established to describe the lifetime decay spectrum expected for positronium (Ps) atoms reacting with a solute by diffusion, taking into account the time dependence of the encounter rate coefficient. To assess the validity of the equations, these are applied to the case of Ps trapping into sodium dodecyl sulfate micellar aggregates in water studied by positron annihilation lifetime spectroscopy (LS). The LS spectra are satisfactorily analysed either on this new basis or in a conventional way, in terms of simple decaying exponentials. Several hypotheses are examined to derive the radius of the micelles from the data, by considering furthermore that the Ps atoms exist in the solution either in a quasi-free or in a bubble state. In complete accordance with the theoretical expectations, the combination of a time dependent encounter rate coefficient together with the bubble state assumption leads to a very good agreement between the radius thus determined (1.72 nm) and the micellar hydrophobic radius known from other techniques (about 1.84 nm).

Positronium-lithium scattering

Abstract: The scattering of positronium (Ps) atoms by lithium (Li) atoms has been studied for the first time. The first Born approximation, the static-exchange approximation and a two-state close-coupling approximation including exchange have been employed to calculate integrated elastic and excitation cross sections and a comparison is made among these three results. It is observed from the results that these approximation methods are sensitive in the lower-energy region, i.e. when the energy (E) of the incident positronium atom is 0.068

Positronium binding to potassium and other alkali atoms

Abstract: The fixed-core stochastic variational method has been used to demonstrate the existence of an electronically stable ground state for potassium positride, KPs, with a binding energy of 0.003 275 Hartree. The 2 annihilation rate of 2.001 × 109 s-1 was almost the same as the annihilation rates computed previously for LiPs and NaPs. Analysis of the radial expectation values, the annihilation rates and the e--e+ correlation function reveal that KPs can be regarded as a Ps cluster moving in the field of the neutral atom.

A critical assessment of the bubble model for positronium annihilation in liquids

Abstract: The bubble model conventionally used to fit the observed characteristics of the pick-off component of ortho-positronium decay in liquids is subjected, in the present study, to a critical assessment. It is demonstrated that in its usual form (namely that of a bubble with a sharp boundary) the model is untenable, when confronted conjointly with experimental data on the lifetime and angular correlation of the decay gammas. A modified version of the model that is relatively free from such shortcomings is presented.

Formation of antihydrogen from antiproton collisions with positronium

Abstract: The cross section for antihydrogen formation in the n = 1, 2 and 3 levels from antiproton-positronium collisions is computed in the unitaiized Bom approximation (uba). Six physical states of antihydrogen (Is, 2s, 2p, 3s, 3p, 3d) and three physical states (Is, 2s, 2p) of positronium are included in the uba basis. The cross section for antihydrogen formation from excited positronium targets is found to be much larger than that from a ground state positronium target at sufficiently low incident energies. This has implications for increasing the production of antihydrogen for planned high resolution spectroscopic experiments.

Decays of Positronium

Abstract: We briefly review the theoretical and experimental results concerning decays of positronium. Possible solutions of the "orthopositronium lifetime puzzle" are discussed. Positronium annihilation into neutrinos is examined and disagreement is found with previously published results.

Low-energy quenching of positronium by helium

Abstract: Very-low-energy scattering of orthopositronium by helium has been investigated for the simultaneous study of elastic cross section and pickoff quenching rate using a model exchange potential. The present calculational scheme, while it agrees with the measured cross section of Skalsey et al., reproduces successfully the parameter 1 Zeff , the effective number of electrons per atom in a singlet state relative to the positron. Together with the fact that this model potential also leads to an agreement with measured medium energy cross sections of this system, this study seems to resolve the long-standing discrepancy at low energies among different theoretical calculations and experimental measurements. @S1050-2947~99!09006-X#