Positron annihilation spectroscopy study of the structure of some liquid molecular media

The structure of a number of liquid molecular media has been investigated by positron annihilation spectroscopy. By the parameters of annihilation spectra, the dimensions of cavities in which the positronium atom is localized have been determined. The possibility of identifying the chemical composition of atoms that form the “wall” of the positronium bubble is discussed.     

Positronium formation in nonpolar solvents. A comparative study of the influence of chlorinated ethanes,ethenes and cyclohexanes on the yield of o-Ps

Inhibition of positronium formation caused by different chlorocyclohexanes, chloroethanes and chloroethenes used as additives in cyclohexane, 2,2,4-trimethylpentane, benzene and n-hexane has been investigated by means of positron lifetime measurements. Chlorocyclohexanes and chloroethanes cause increased inhibition with increasing chlorine substitution. Comparisons with earlier results of chlorobenzenes show that the behaviour of the actual additives is quite different from that of the chlorobenzenes. The results are discussed in terms of the “hot Ps-model” and the “spur-model”.     

Convergence of the orbital expansion in a correlated system: A test study on positronium

Positronium, the bound state of an electron and a positron, is an exactly soluble quantum system, similar to a light isotope of hydrogen. It can be studied using the finite basis quantum chemistry codes developed for atoms and molecules. In fact, positronium can be mimicked by two electrons with opposite spins, in the absence of any nucleus and having the sign of the Coulomb interaction reversed. The exact wave function has a cusp in the points of coalescence of the two particles (a “Coulomb peak”), and this fact makes the convergence of the total energy, as a function of the basis set size, extremely slow. For this reason, positronium can be used to test the convergence properties of the quantum chemistry methods used to describe the dynamic correlation. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Positronium formation affected by intermolecular electron transfer in concentrated solutions of electron acceptors

The positronium yield has been measured in mixtures of electron acceptors (inhibitors and anti-inhibitors of positronium formation) diluted with a neutral solvent. The data obtained confirm the idea, based on the spur reaction model of positronium formation, that the charge transfer reaction between electron acceptors in the positron spur is of importance for positronium formation.     

Multireference configuration interaction calculations for positronium halides

Multireference configuration interaction (MRCI) calculations of the positronium halides, PsF, PsCl, PsBr, and PsI, are carried out, to give positron ionization energies, positronium binding energies, and two-photon annihilation rates. All CI calculations consider only valence correlation effect with a frozen-core approximation, and use the orbitals with angular momentum up to 8. To incorporate the effects of many-body correlations in the energies and two-photon annihilation rates, the MRCI calculations are repeated with increasing reference configurations, and the full CI limits of these energies and annihilation rates are estimated. The contribution from orbitals having angular momentum greater than 8 to those values is also estimated. Relative to our previous single reference CI calculations, many-body correlation effects significantly increase the positron ionization energies, positronium binding energies, and two-photon annihilation rates. The structures of the positronium halides are also discussed.     

Correlation effects in positron-electron systems: A Quantum Monte-Carlo study

We present Quantum Monte-Carlo results for various positron-electron systems. We discuss the energetics of the bound state of a positronium (Ps) with finite atomic systems: a massive point charge Z, hydrogen (H) and lithium (Li). We also analyze the screening cloud around the positron in the positronium hydride (HPs) and in the homogeneous electron gas in order to determine the positron annihilation rates.     

Theoretical study of positron scattering by group 14 tetra hydrides: A quantum mechanical approach

The present work provides a comprehensive set of positron impact scattering cross sections for group 14 tetrahydrides, namely, SiH₄, GeH₄, SnH₄, and PbH₄. The well‐established spherical complex optical potential and complex scattering potential‐ionization contribution methods are modified to incorporate positron scattering in the present work to calculate various cross sections. The positronium formation channel is adequately included through an improved inelastic threshold. The energy range chosen for the direct ionization cross section is from the respective ionization potential (I) of the molecule to 5 keV. Likewise, positronium formation and total ionization cross sections are reported from the positronium formation threshold to 300 eV and 5 keV, respectively, and the total cross section is computed for an extensive energy range of 1 eV to 5 keV. The positron impact total cross section for stannane molecule is computed for the first time. A characteristic valley is observed in the total cross sections with minima close to the positronium formation threshold. Further increase of cross section signifies the opening of inelastic channels especially positronium formation. In general, a reasonable agreement is found between the present results and other comparisons, wherever available. Furthermore, this is the first report of the inelastic cross sections (direct ionization, positronium formation, and total ionization) for the present set of targets.     

The interaction of slow positrons with organic moecules above and below positronium formation thresholds

A review is given of the ionization of organic moecules by monoenergetic positrons having energies in the range of 0.5–15 eV. Two mechanisms, unique to positrons, are described. If the kinetic energy of the positron is above the positronium formation threshold, such that electrons can be removed from the molecules to form free positronium atoms, the ionization/fragmentation behavior can be explained qualitatively by a modification of the Ore gap theory. To explain how positrons can ionize and fragment molecules when their kinetic energies are below the positronium formation threshold, it is necessary to assume that energy is transferred to the molecule by the annihilation process. Ionization cross sections for positrons having kinetic energies below the positronium formation threshold are sensitive to molecular size, structure and bond types. Continuing work involves a search for positronium compound formation and measurements of the kinetic energy distributions of ions.     

A sense of new beginnings in positron and positronium chemistry

Conclusion This has been the most future-oriented workshop in my memory. Many participants are thinking about the next experiments to perform, new phenomena to study, and new parameters to measure. We look forward to new optical correlations, both with exciting radiation and intrinsic fluorescence. We very badly need positron and positronium binding energies to atoms, molecules, and radicals. We very badly need work functions, band gaps and edges, and trap depths for positron and positronium in molecular liquids and solids. The binding energies of positronium itself in various molecular materials is an important and poorly understood quantity. New data presented at this workshop on the energy of positronium at its formation, and its thermalization time, are most significant,³ and we look forward to refinements in those measurements and in their interpretation. We need to understand the thermalization times of positrons much better than we do. The role of voids and/or surfaces in positronium formation needs to be illuminated, including numerical estimates and measurements for the cross section of this most important process.Finally, a note of thanks and appreciation to the hard-working organizers of this workshop, without whose selfless labors we would not have enjoyed the many benefits of talking face to face with our colleagues from far away.     

A non-conventional method of polymer PAL spectrum analysis by the LT computer program

A new version of computer program (LT v.10) offers a new concept of polymer spectrum analysis. The theoretical model takes into account a few processes in which positron is involved: the process of slow formation and localization of positronium (Ps), delay formation of positronium from shallow trapped electrons and positron trapping in irradiation-induced centers. The model was applied to two series of spectra for LDPE and HDPE collected at constant temperature (much below the glass temperature) as a function of measurement time. The Ps internal relaxation time and time of localization of Ps in a free-volume center were determined. The results show that the trapping rate of positron is strictly correlated with the amount of the shallow trapped electrons. That suggests a coupling between the positron and electron-trapping centers.     

Slow positron study on dielectric-coated mirror for free-electron-laser experiments

A positron lifetime study has been done on dielectric multilayer cavity mirrors for free-electron-laser experiments by the use of a variable-energy pulsed positron beam. A long-lived ortho-positronium component has been observed at low positron energy region, corresponding to the depth of the top amorphous SiO₂ layer. The intensity of the positronium component correlates with the degradation and restoration of the mirrors. We discuss the relation between the positronium intensity and degradation mechanism of the mirrors. The present studies revealed that the slow positron lifetime technique is highly sensitive to the properties of the mirrors and is useful for the evaluation of the mirrors.     

Positron interactions with overlayers of oxygen on GaAs(100)

The positron work-function, re-emission yield, and positronium fraction of an n-doped GaAs(100) surface were measured as a function of oxygen exposure. The energy distribution of positrons observed to be re-emitted indicated that the clean and oxygen exposed n-doped GaAs(100) surfaces had negative positron work-functions. The fraction of incident positrons re-emitted as bare positrons, (Y), was found to increase and the fraction re-emitted as positronium, (f_Ps), to decrease with increasing oxygen exposure. This suggests that surface modified GaAs may be useful as a contact material in the fabrication of GaAs based FAMs.     

A positron lifetime spectroscopy apparatus for surface and near-surface positronium experiments

A positron lifetime spectroscopy apparatus, which utilizes an intense pulsed positron beam generated by an electron linear accelerator, is presented. Using this apparatus we can measure positron lifetime spectra with variable energy, high count rate, wide measurable time range, and high peak-to-background ratio. These features are suitable for surface and near-surface positronium experiments.     

Low-temperature formation of positronium in nonpolymer organic systems

The phenomenon of the low-temperature formation of positronium (bound electron-positron system) observed during positron irradiation of a test substance is discussed. The effect is additional relative to the formation of positronium under normal conditions and, like the principal phenomenon, is radiation-chemical in nature. This effect has been discovered and studied mainly for polymers, in which the role of weakly bound electrons and positron localization (trapping) in the intensity of manifestation of this phenomenon was revealed. This review focuses on studies concerning barely investigated nonpolymer (polar and nonpolar) organic systems, in which a relatively high mobility of molecules as compared to polymers can open an opportunity for a more pictorial manifestation of positron and electron trapping by polar centers, both belonging to separate molecules and resulting from collective orientation of the molecules.     

Positronium yields in amorphous,cross-linked and conductive polystyrene

Variations in positronium yields due to positron irradiation of specimens during experiment were investigated on the three commercially available modifications of polystyrene (Goodfellow): amorphous, cross-linked and conductive. Positron lifetime technique was employed. The variations of the positronium yields were expressed as changes of the ortho-positronium intensity as functions of the irradiation time. It was found that the positronium yield curves obtained for the amorphous and cross-linked polystyrene cannot be represented as a simple single-exponential relaxation towards a steady state and at least one additional component or a modified shape of the relaxation curve should be considered.     

An application of the coincidence Doppler spectroscopy for substances of chemical interest: phthalocyanine and acetylacetonate complexes

Coincidence Doppler spectroscopy, which is particularly powerful when one is concerned with high momentum components of positron annihilation gamma-rays, has been applied to two different kinds of organo-metalic ligands: metal phthalocyanines and metal acetylacetonates. The energy (momentum) profiles of the annihilation gamma-rays were the same for metal phthalocyanines indicating that positron and/or positronium are not interacting with the metal ions. However, the profiles for the metal acetylacetonates evidently showed a dependence on the kind of metal ions. Discussion is made on the features of positron interaction which are different for phthalocyanines and acetylacetonates.     

Influence of Chemical Reactions of Positronium on Annihilation Characteristics in Polymeric Sorbents

Size distribution of free volume holes for several types of polymeric sorbents based on crosslinked polystyrene was studied using the positron annihilation and Brunauer–Emmett–Teller low-temperature nitrogen sorption techniques. It turned out that the ranges of free volume size detectable by the positron and sorption techniques partially overlap, wherein the positron annihilation technique covers the region of nanopores and the sorption method covers submicronic pores and a part of nanopores. Thus, both methods well complement one another. Information obtained by means of the positron technique can be confused if the positronium atoms produced by positrons injected in a material of interest take part in chemical reactions. Cases when such reactions can be suppressed by lowering the temperature are discussed.     

The design of a spin-polarized slow positron beam

Spin-polarized low-energy positrons were considered as useful probes for studying electron spin states of both surface and bulk materials. Due to the spin-dependent interactions between electrons and positrons, the formation of positronium (Ps), an electron-positron bound system, can be distinguished from different electron spin states. Recently, a positron source of ¹⁸F has been developed for a spin polarized slow positron beam at the institute of physical and chemical research (RIKEN). The design of an electrostatic positron beam will be discussed in conjunction with a spin rotator.     

Chemical reactions of positronium studied by age-momentum correlation (AMOC) using a relativistic positron beam

Correlated measurements of the individual positron lifetimes (so-called positron ages) and of the Doppler shift of the 2-annihilation photon energy (Age-Momentum Correlation, AMOC) allow the evolution of the positron states to be observed in the time domain. The AMOC technique is thus especially useful when chemical reactions of positronium induce transitions between positron states. The full information contained in the AMOC data may be extracted from the two-dimensional AMOC histogramme, the so-called AMOC relief, by fitting a suitable two-dimensional model function. In this way quantitative information on the spin conversion of positronium by a paramagnetic radical in the systems HTEMPO/methanol and HTEMPO/benzene and on the formation of positron bound states in aqueous solutions of various sodium halides has been deduced.