Showing papers on "Positronium published in 2002"

Positron and positronium binding to atoms

Abstract: Recent research has shown that there are a number of atoms and atomic ions that can bind a positron. The number of atoms known to be capable of binding a positron has expanded enormously in recent years, with Li, He(3Se), Be, Na, Mg, Ca, Cu, Zn, Sr, Ag and Cd all capable of binding a positron. The structure of these systems is largely determined by the competition between the positron and the nucleus to bind the loosely bound valence electrons. Some systems, such as e+Li and e+Na, can be best described as a Ps cluster orbiting a charged Li+ or Na+ core, while others such as e+Be consist of a positron orbiting a polarized Be atom. In addition, a number of atoms (Li, C, O, F, Na, Cl, K, Cl, Cu, Br) can bind positronium and a few systems capable of binding two positrons have also been identified. These positron-binding systems decay by electron-positron annihilation with the annihilation rate for e+A systems largely determined by the parent atom ionization potential.

Two-center convergent close-coupling approach to positron-hydrogen collisions

Abstract: The question of convergence in two-center close-coupling expansions is addressed via the study of positron-hydrogen scattering. It is found that the major cross sections do converge if sufficient number of states from a complete basis, centered separately on the hydrogen atom and positronium, are used to expand the total scattering wave function. The underlying equations are computationally highly ill conditioned, and a simple, numerically efficient technique is given that alleviates the problem. Generally, we find good agreement with available experiment and some previous theory. However, calculations that only used eigenstates for the positronium center yielded cross sections for positronium formation in the $2s$ and $2p$ states that are higher than the convergent ones obtained in this work.

Total positron-impact ionization and positronium formation from the noble gases

Abstract: Detailed measurements of the total ionization cross-sections for Ne, Ar, Kr and Xe by positron impact are presented in the energy range from threshold up to ~1 keV. From these and other available data, the cross-sections for positronium formation are extracted. Comparison is made with other results where available. Structure in the positronium formation cross-section has been found which appears to be associated with positronium formation in excited states. Upper and lower limits on the contribution from these are estimated.

Positronium molecule formation, Bose–Einstein condensation and stimulated annihilation

Abstract: Low energy positron production and storage may be used to make and observe some interesting many-positron, many-electron systems both in vacuum and in the presence of ordinary matter. One of the most interesting possibilities is to make an annihilation photon laser. One might start on the path to this laser by creating a high density positron burst and forming a dense gas of positronium atoms within a cavity in a solid. This will pave the way for subsequently demonstrating the near room temperature Bose–Einstein condensation of a dense gas of positronium and establishing conditions under which stimulated emission of annihilation photons can be observed.

Can we measure the gravitational free fall of cold Rydberg state positronium

Abstract: In this paper we examine the possibilities for detecting the free fall of Rydberg positronium atoms. In our scheme, cold positronium atoms are emitted from a “point” source and excited to the n=25 circular Rydberg state with L=n−1. The positronium atoms are allowed to travel horizontally 10 m in a field free vacuum and focused onto a detector using an elliptical Van der Waals mirror. A free fall distance of order 50 μm and a few detected atoms per hour are anticipated. Various extraneous influences on the positronium, such as collisions with residual gas atoms, Stark mixing in stray electric and magnetic fields, photoionization due to thermal radiation, and accelerations due to patch potentials are estimated.

Positron Annihilation Study of Hyper-Cross-Linked Polystyrene Networks

Abstract: Hyper-cross-linked polystyrene sorbents with various degree of cross-linking ranging from 25% to over 100% were studied using positron annihilation lifetime (PAL) spectroscopy. Long-lived components of the lifetime distribution of positron annihilation, orthopositronium (o-Ps) lifetimes, give information on the effective size of elementary free volumes (unoccupied spaces), responsible for the properties of sorbents. Experiments were carried out in a vacuum, in air and also in oxygen and nitrogen. Narrow size distribution of elementary free volumes having radius of about 1.5 nm and concentration estimated as ∼1019 cm-3, were found in the samples with cross-linking higher than 40%. Some of the hyper-cross-linked samples gave positronium lifetime τ5 > 50 ns, which is among the highest values ever measured for organic polymer compounds. Positronium quenching (shortening of o-Ps lifetime) due to Ps exchange interaction with oxygen, dissolved in sorbents, gave information about the character of distribution of ...

Fragmentation of positronium in collision with He atoms.

Abstract: The absolute cross section for the fragmentation of positronium in collision with He atoms has been measured. The results are compared with available theories. The longitudinal energy distributions of positrons resulting from fragmentation have also been determined and are found to display a peak situated just below half the residual energy. This is suggestive of the occurrence of "electron loss to the continuum" in which the two residual charged particles lie in a low relative-velocity Coulomb-continuum state.

Nanoporous structure of sputter-deposited silicon oxide films characterized by positronium annihilation spectroscopy

Abstract: Positronium annihilation was applied to characterize the nanoporous structure of thin silicon oxide films sputter-deposited at different argon pressures ranging from 0.1 to 2.0 Pa. At higher argon pressures, the 3γ decay probability of ortho-positronium (o-Ps) was substantially enhanced. A comparison of this result with that obtained for capped samples indicated that: (a) 3γ annihilation is due to the intrinsic decay of o-Ps diffusing out from the film into vacuum and (b) films deposited at high argon pressures contain highly connected, open pores. Positron lifetime spectroscopy measurements on the capped films showed that the characteristic size of the pores can be as large as 2.6 nm, depending on the argon pressure.

Positronium scattering by atomic hydrogen with inclusion of target excitation channels

Abstract: Calculations are reported for positronium (Ps) scattering by atomic hydrogen (H) in the energy range 0--6.5 eV in a coupled-pseudostate approximation in which excitation and ionization channels of both the Ps and the H are taken into account. The approximation contains an accurate representation of the van der Waals coefficient. Results are presented for phase shifts, scattering lengths, effective ranges, and various cross sections including partial wave, total, and ortho-para conversion cross sections. An analysis of the possible spin transitions is provided and the energy of the positronium hydride (PsH) bound state is determined. Substantial differences are found from earlier work within the frozen target approximation, now clearly confirming the importance of target excitation channels. Good agreement is obtained with recent calculations of S-wave phase shifts and scattering lengths using the stabilization method. Convergence to the exact binding energy for PsH appears to be slow. Resonances corresponding to unstable states of the positron orbiting ${\mathrm{H}}^{\ensuremath{-}}$ are seen in the electronic spin singlet partial waves. The importance of the ${\mathrm{H}}^{\ensuremath{-}}$ formation channel is discussed.

Anti-Matter Wave Interferometry with Positronium

Abstract: Atomic matter wave optics is a well-developed field in physics. Many different coherent manipulation techniques for atoms have been developed. In this paper the application of one of these techniques, Bragg scattering at near-resonant standing light waves, for ortho-positronium is discussed. Utilizing Bragg diffraction as a coherent splitting mechanism the realization of a Positronium interferometer is discussed. Estimates for the measurement of gravitational acceleration for Positronium are given.

Electric field effect on positronium formation in liquids

Abstract: The comparison of different models (the Ore, spur, and blob models) of positronium (Ps) formation is presented. Because in molecular media Ps is formed in the terminal positron blob and not in an ordinary spur, the application of the blob model seems to be the most adequate. We extend this model for consideration of the Ps formation in the presence of an external electric field (<100 kV/cm). In the simplified limiting case, this approach provides a formula similar to the Onsager one for the geminate recombination probability. The influence of ion–electron recombination and other intrablob processes on Ps formation is taken into account. The role of quasifree positronium in the Ps formation process is discussed.

Depth-profiling plasma-induced densification of porous low-k thin films using positronium annihilation lifetime spectroscopy

Abstract: Positronium annihilation lifetime spectroscopy (PALS) has been used to depth profile the densification induced in a porous low-dielectric constant (k) thin film by typical device integration processing, including exposure to plasmas and oxygen ashing. Such “integration damage” has previously been observed as an undesirable increase in k accompanied by shrinkage in the porous film thickness. PALS confirms that the structural damage is confined to a surface layer of collapsed pores with the underlying pores being undamaged. The dense layer thickness determined by PALS increases with plasma exposure time.

Hydrogen: The Essential Element

Abstract: Prologue 1 In the Beginning: Hydrogen and the Big Bang 2 Hydrogen and the Unity of Matter: The Prout Hypothesis William Prout, 1815 3 Hydrogen and the Spectra of the Chemical Elements: A Swiss High School Teacher Finds a Pattern Johann Jakob Balmer, 1885 4 The Bohr Model of Hydrogen: A Paradigm for the Structure of Atoms Niels Bohr, 1913 5 Relativity Meets the Quantum in the Hydrogen Atom Arnold Sommerfeld, 1916 6 The Fine-Structure Constant: A Strange Number with Universal Significance Arnold Sommerfeld, 1916 7 The Birth of Quantum Mechanics: The Hydrogen Atom Answers the "Crucial Question" Werner Heisenberg and Wolfgang Pauli, 1925-26 * Paul Dirac, 1925-26 8 The Hydrogen Atom: Midwife to the Birth of Wave Mechanics Erwin Schrodinger, 1926 9 The Hydrogen Atom and Dirac's Theory of the Electron Paul Dirac, 1928 10 Hydrogen Guides Nuclear Physicists: The Discovery of Deuterium Harold Urey, 1932 11 Hubris Meets Hydrogen: The Magnetic Moment of the Proton Otto Stern, 1933 12 The Magnetic Resonance Method: The Origin of Magnetic Resonance Imaging I I Rabi, 1938 13 New Nuclear Forces Required: The Discovery of the Quadrupole Moment of the Deuteron Norman F Ramsey and I I Rabi, 1939 14 Magnetic Resonance in Bulk Matter (NMR) Edward M Purcell and Felix Bloch, 1946 15 Hydrogen's Challenge to Dirac Theory: Quantum Electrodynamics as the Prototype Physical Theory Willis Lamb, 1947 16 The Hydrogen Atom Portends an Anomaly with the Electron I I Rabi, John E Nafe, and Edward B Nelson, 1946 17 Hydrogen Maps the Galaxy Edward M Purcell and Harold Ewen, 1951 18 The Hydrogen Maser: A High-Precision Clock Norman F Ramsey and Daniel Kleppner, 1960 19 The Rydberg Constant: A Fundamental Constant Johannes Robert Rydberg, 1890 * Theodor Hansch, 1992 20 The Abundance of Deuterium: A Check on Big Bang Cosmology David N Schramm, 1945-1997 21 Antihydrogen: The First Antiatom 22 The Bose-Einstein Condensate for Hydrogen Satyendranath Bose, 1924 * Albert Einstein, 1925 * Eric A Cornell and Carl E Wieman, 1995 * Daniel Kleppner and Tom Greytak, 1998 23 Exotic Hydrogen-like Atoms: From Theory to Technology Epilogue Notes Acknowledgments Credits Index

Positronium scattering by Ne, Ar, Kr and Xe in the frozen target approximation

Abstract: A full-electron coupled-state treatment of positronium (Ps)-inert gas scattering is developed within the context of the frozen target approximation. Calculations are performed for Ps(1s) scattering by Ne and Ar in the impact energy range 0-40 eV using coupled pseudostate expansions consisting of nine and 22 Ps states. The purpose of the pseudostates is primarily to represent ionization of the Ps which is found to be a major process at the higher energies. First Born estimates of target excitation are used to complement the frozen target results. The available experimental data are discussed in detail. It is pointed out that the very low energy measurements (≤2 eV) correspond to the momentum transfer cross section σmom and not to the elastic cross section σel. Calculation shows that σmom and σel diverge very rapidly with increasing energy and consequently comparisons of the low-energy data with σel can be very misleading. Agreement between the calculations and the low-energy measurements of σmom, as well as higher energy (≥15 eV) beam measurements of the total cross section, is less than satisfactory. Results for Ps(1s) scattering by Kr and Xe in the static-exchange approximation are also presented.

Resonances of positronium complexes

Abstract: Resonances of electron-positron complexes are studied by the stochastic variational method combined with the complex scaling method. Several low-lying resonances in the positronium negative ion are obtained. One P-state and nine S-state resonances of the positronium molecule are predicted below the Ps(n=1) +Ps(n=4) threshold. The structure of some of these resonances is analyzed.

Calculation of cross sections for very low-energy hydrogen-antihydrogen scattering using the Kohn variational method

Abstract: In view of current interest in the trapping of antihydrogen () atoms at very low temperatures, we have carried out a calculation of s-wave hydrogen–antihydrogen scattering at very low energies, using the Kohn variational method, taking into account rearrangement scattering into the three channels that contain positronium in its ground state and lie closest to threshold. We find that our values for the elastic cross section are in good agreement with the values obtained by Jonsell et al (Jonsell et al 2001 Phys. Rev. A 64 052712) using a distorted wave approximation. However, our values for the total rearrangement cross section are much larger than their values. In particular, the largest such cross section is for the N = 23 s-state of protonium and positronium in its ground state, a channel that was estimated to make a negligible contribution by Jonsell et al. As a consequence of our much larger values for the total rearrangement cross section, we predict that cooling of by cold H would be considerably less efficient than was found to be the case by Jonsell et al.

Positronium formation in e + − Li and e + − Na collisions at low energies

Abstract: Measurements of positronium formation cross sections for positrons in the energy range down to a few tenths of one electron volt scattered by Li and Na atoms are reported. The reasonable agreement of the measured cross sections for Li with theoretical predictions is in striking contrast to the case for Na where there is a pronounced divergence between calculated and measured values below 1 eV.

Positronium trapping in small voids: Influence of their shape on positron annihilation results

Abstract: Positron annihilation lifetime spectroscopy is extensively used to get information on the sizes of the nanoholes existing in porous and nonmetallic solids. Cavities are most often modeled as spherical, although their shape is irregular. In the present paper the relationship between lifetime distribution as extracted from a time annihilation spectrum and size (and volume) distributions of the holes for some nonspherical geometries is discussed. It is shown that the same lifetime distribution can produce cavity size distributions having different momenta (centroid and standard deviation), if the shape of the hole is modified. Nevertheless, the ratio between the two momenta is largely independent of the geometry. Furthermore, parameters derived from the positron measurements (such as the free volume fraction) are not strongly influenced by the choice of the hole geometry.

Positronium-positronium scattering using the stochastic variational method

Abstract: The stochastic variational method is used in conjunction with stabilization ideas to compute the low-energy s-wave phase shifts and scattering lengths for Ps-Ps scattering. The scattering lengths obtained were 8.44a0 for the singlet-singlet and 3.00a0 for the triplet-triplet states (the spin state refers to the coupling of the two electrons and the coupling of the two positrons in the system). The positive scattering length in the triplet-triplet channel means a stable Bose-Einstein condensate of triplet Ps atoms is physically possible. The large value of the 3Ps3Ps→1Ps1Ps cross section, 7.41πa02 suggests the use of spin-polarized positrons as advanced by Platzmann and Mills [Phys. Rev. B 49 454 (1994)].

General trends of positronium pick-off annihilation in molecular substances

Abstract: The characteristics of positronium (Ps) annihilation in molecular substances (ranging from organic liquids to molecular solids), manifested through the observed lifetimes (τp) for the 'pick-off' process and values of angular correlation (θ1/2) of decay gammas, can be shown to be simply related to the size (radius R) of the cavity which the Ps creates in a liquid or finds in a molecular solid. The measured τp and θ1/2 are in turn calculable from the wavefunction describing the Ps centre-of-mass motion, which is determined from the average potential experienced by it in the confining cavity. Thus the height of this repulsive barrier (U0) at the wall of this vacant region corresponding to different materials (with varying R) can be obtained by fitting experimental observations, namely τp and θ1/2. The model we use is an improved version of the usual spherical well description of the cavity (where we take the walls to be diffuse). It is found that the values of U0 and R, taking into account all available and relevant data for different molecular substances, fall on a universal curve. We attempt to explain the reason behind this 'universality' by relating the potential to the Ps work-function in materials. Finally, the fit provides us with a very convenient linear relationship between the size (R) of the cavity and the pick-off lifetime (τp).

Positron and positronium chemistry by quantum Monte Carlo. VI. The ground state of LiPs, NaPs, e+Be, and e+Mg

Abstract: The ground states of the positronic complexes LiPs, NaPs, e+Be, e+Mg, and of the parent ordinary-matter systems have been simulated by means of the all-electron fixed-node diffusion Monte Carlo (DMC) method. Positron affinities and positronium binding energies are computed by direct difference between the DMC energy results. LiPs was recomputed in order to test the possibility of approximating the electron–positron Coulomb potential with a model one that does not diverge for r=0, finding accurate agreement with previous DMC results. As to e+Be, the effect due to the near degeneracy of the 1s22s2 and 1s22p2 configurations in Be is found to be relevant also for the positron affinity, and is discussed on the basis of the change in the ionization potential and the dipole polarizability. The DMC estimate of the positron affinity of Mg, a quantity still under debate, is 0.0168(14) hartree, in close agreement with the value 0.015 612 hartree computed by Mitroy and Ryzhihk [J. Phys. B. 34, 2001 (2001)] using expl...

Positron and positronium interactions with Cu

Abstract: The configuration-interaction (CI) method is used to investigate the interactions of positrons and positronium with copper at low energies. The calculations were performed within the framework of the fixed-core approximation with semiempirical polarization potentials used to model dynamical interactions between the active particles and the (1s-3d) core. Initially, calculations upon the e{sup +}Li system were used to refine the numerical procedures and highlighted the extreme difficulties of using an orthodox CI calculation to describe the e{sup +} Li system. The positron binding energy of e{sup +}Cu derived from a CI calculation which included electron and positron orbitals with l{<=}18 was 0.005 12 hartree while the spin-averaged annihilation rate was 0.507x10{sup 9} s{sup -1}. The configuration basis used for the bound-state calculation was also used as a part of the trial wave function for a Kohn variational calculation of positron-copper scattering. The positron-copper system has a scattering length of about 13.1a{sub 0} and the annihilation parameter Z{sub eff} at threshold was 72.9. The dipole polarizability of the neutral copper ground state was computed and found to be 41.6a{sub 0}{sup 3}. The structure of CuPs was also studied with the CI method and it was found to have a binding energy ofmore » 0.0143 hartree and an annihilation rate of {approx}2x10{sup 9} s{sup -1}.« less

Hyperfine structure of the ground and first excited states in light hydrogen-like atoms and high-precision tests of QED

Abstract: We consider hyperfine splitting of 1s and, in part, of 2s levels in light hydrogen-like atoms: hydrogen, deuterium, tritium, helium-3 ion, muonium and positronium. We discuss present status of precision theory and experiment for the hfs intervals. We pay a special attention to a specific difference, D 21=8E hfs(2s)−E hfs(1s), which is known experimentally for hydrogen, deuterium and 3He+ ion. The difference is weakly affected by the effects of the nuclear structure and thus may be calculated with a high accuracy. We complete a calculation of the fourth order QED contributions to this difference and present here new results on corrections due to the nuclear effects. Our theoretical predictions appear to be in a fair agreement with available experimental data. Comparison of the experimental data with our examination of D 21 allows to test the state-dependent sector of theory of the hfs separation of the Is and 2s levels in the light hydrogen-like atoms up to 10−8.

Enhancement of positron-atom annihilation near the positronium formation threshold

Abstract: The behavior of the positron- 2 gamma annihilation rate on an atomic target near the positronium (Ps) formation threshold is determined. When the positron energy epsilon approaches the threshold epsilon(thr) from below, the effective number of electrons contributing to the annihilation, Z(eff), grows as Z(eff) approximately A/square root of [epsilon(thr)-epsilon], where A is related to the size of the Ps formation cross section, sigma(Ps) approximately B square root of [epsilon-epsilon(thr)], by A = B square root of [2 epsilon(thr)]/32 pi (in atomic units). Taking account of the finite Ps lifetime eliminates the singularity in Z(eff) and shows that close to threshold the positron annihilation cross section is identical to the para-Ps formation cross section.

Study on laser cooling of ortho-positronium

Abstract: We have been theoretically and experimentally studying details of laser cooling of ortho-positronium. Experimental apparatus consists of a pulsed positron beam generator, a long pulse wide bandwidth ultraviolet laser and a time-of-flight system to measure kinetic energy of positronium atoms is constructed and tested. Using this apparatus, production of thermally activated positronium is confirmed. Its production rate is larger as the target temperature increases and ratio of thermally activated ortho-positronium to all γ-ray annihilation events is estimated to be 32.3±6.7% for the target temperature 1000 K.