Positron Correlation Research Articles

Calculation of Low-Energy Positron-Atom Scattering with Square-Integrable Wavefunctions

The variational method is applied to the low-energy positron scattering and annihilation problem. The ultimate aim of the investigation is to find a computationally economical way of accounting for strong electron–positron correlations, including the effect of virtual positronium formation. The method is applied to the study of elastic s-wave positron scattering from a hydrogen atom. A generalized eigenvalue problem is set up and solved to obtain s-wave positron–hydrogen scattering phase shifts within 8×10−3 rad of accepted values. This is achieved using a small number of terms in the variational wavefunction; in particular, only nine terms that depend on the electron–positron distance are included. The annihilation parameter Zeff is also calculated and is found to be in good agreement with benchmark calculations.

Correlated Wave Functions for Electron-Positron Interactions in Atoms and Molecules.

The positron, as the antiparticle of the electron, can form metastable states with atoms and molecules before its annihilation with an electron. Such metastable matter–positron complexes are stabilized by a variety of mechanisms, which can have both covalent and noncovalent character. Specifically, electron–positron binding often involves strong many-body correlation effects, posing a substantial challenge for quantum-chemical methods based on atomic orbitals. Here we propose an accurate, efficient, and transferable variational ansatz based on a combination of electron–positron geminal orbitals and a Jastrow factor that explicitly includes the electron–positron correlations in the field of the nuclei, which are optimized at the level of variational Monte Carlo (VMC). We apply this approach in combination with diffusion Monte Carlo (DMC) to calculate binding energies for a positron e+ and a positronium Ps (the pseudoatomic electron–positron pair), bound to a set of atomic systems (H–, Li+, Li, Li–, Be+, Be, B–, C–, O– and F–). For PsB, PsC, PsO, and PsF, our VMC and DMC total energies are lower than that from previous calculations; hence, we redefine the state of the art for these systems. To assess our approach for molecules, we study the potential-energy surfaces (PES) of two hydrogen anions H– mediated by a positron (e+H22–), for which we calculate accurate spectroscopic properties by using a dense interpolation of the PES. We demonstrate the reliability and transferability of our correlated wave functions for electron–positron interactions with respect to state-of-the-art calculations reported in the literature.

Zr vacancies and their complexes with hydrogen in monoclinic zirconia: formation energies and positron lifetimes

Cation vacancies in metal oxides have high formation energies and, thus, are not as abundant as their oxygen-related counterparts. Nonetheless, they can be readily created during non-equilibrium processes. Positron-annihilation spectroscopy (PAS) is a well suited technique of probing such defects since negatively-charged lattice vacancies are deep potential wells which can act as positron traps. The present study reports first-principles calculations of positron trapping at Zr monovacancies in monoclinic zirconia. The binding of positrons and associated lifetimes for these defects were obtained within two-component density-functional theory under different approximations for the electron–positron correlation. The role of hydrogen was also explored. Low-energy vacancy–hydrogen defect complexes were determined for one and two hydrogen atoms bound to a single Zr monovacancy. Hydrogen decoration of the vacancy affected the localization of the positron leading to an appreciable decrease of the lifetime. The study was supplemented with PAS measurements on samples of monoclinic zirconia. From the PAS data two distinct, high-intensity components were resolved originating from different defect states. The corresponding lifetimes of 187 and 225 ps were very close to the theoretical predictions.

Do positrons measure atomic and molecular diameters?

We report on density functional calculations (DFT) of elastic integral scattering cross-sections for positron collisions with argon, krypton, nitrogen and methane. The long-range asymptotic polarization potential is described using higher-order terms going much beyond an induced dipole potential (−α / r 4) while the short-range interaction is modeled by two different forms of electron – positron correlation potential (Boronski-Nieminen and Quantum Monte Carlo potentials). The results of both approaches agree quite well with the recent theoretical and measured values. Based on the present and previous theoretical and experimental data we discuss some systematics observed in integral scattering cross-sections below the positronium formation threshold. In particular we point out on the correlation between the values of scattering cross-sections and atomic dimensions.

Insight into the electron–positron correlations in metals through the looking glass

A semi-empirical analysis of the positron annihilation experimental spectra indicates for a strong sensitivity of the two-particle electron–positron (e–p) enhancement factor to the l=s, p, d, f character of the initial electronic state [1,2]. The essential discrepancy between the models consists in the dependence of the relevant correlation functions on the energy of the annihilating electron. The present contribution contains a theoretical study of the e–p enhancement factors for s, p, d and f states as a function of the electron energy. The slope of the resulting characteristics is directly related to the degree of localisation of the s, p, d and f electrons in the electron density of states. This effect occurs especially for d electrons in transition metals, in favour to the approach of Ref. [1].The energy dependence of the two-particle correlation functions is also a source of controversy between various theoretical approaches. The energy dependent enhancement factors describe properly the positron interaction with delocalised s and p electrons, but this approach overestimates the high momentum components of the e–p momentum densities, dominated by the localised d and f states. On the contrary, the calculations that employ the energy averaged enhancement factors match better with experiment for localised d and f electrons, but they hardly reproduce experimental spectra for nearly-free electron populations. An attempt to visit two sides of the looking glass is made in the theory of the present work. The model combines the properties of both approaches. The resulting e–p momentum densities and enhancement factors are in good agreement with the experimental data for simple, noble and transition metals, both in the low and high momentum region.

Effect of dipole polarizability on positron binding by strongly polar molecules

A model for positron binding to polar molecules is considered by combining the dipole potential outside the molecule with a strongly repulsive core of a given radius. Using existing experimental data on binding energies leads to unphysically small core radii for all of the molecules studied. This suggests that electron–positron correlations neglected in the simple model play a large role in determining the binding energy. We account for these by including the polarization potential via perturbation theory and non-perturbatively. The perturbative model makes reliable predictions of binding energies for a range of polar organic molecules and hydrogen cyanide. The model also agrees with the linear dependence of the binding energies on the polarizability inferred from the experimental data (Danielson et al 2009 J. Phys. B: At. Mol. Opt. Phys. 42 235203). The effective core radii, however, remain unphysically small for most molecules. Treating molecular polarization non-perturbatively leads to physically meaningful core radii for all of the molecules studied and enables even more accurate predictions of binding energies to be made for nearly all of the molecules considered.

Positron Annihilation Study of Defects in the Cage-Type Uranium Compound

The electron and positron properties of the UIr2Zn20 compound of the Fd-3m cage-type crystal structure are studied. The characteristics for the perfect crystal are compared with their counterparts in the elements containing Ir and Zn vacancy. The presence of the vacancy hardly in uences the electron charge density around individual atoms. In contrast to electrons, the positron is strongly localized inside the vacancy and the transfer of positron charge is observed. The positron lifetime, τ , appears to be strongly sensitive to the presence and type of the vacancy. Furthermore, this parameter calculated for the perfect crystal is very close to the experimental value for Zn. Here the theoretical values of τ depend on the approach applied to the electron positron (e p) correlations.

Phase space factors forβ+β+decay and competing modes of double-βdecay

A complete and improved calculation of phase space factors (PSF) for $2\nu\beta^+\beta^+$ and $0\nu\beta^+\beta^+$ decay, as well as for the competing modes $2\nu EC\beta^+$, $0\nu EC\beta^+$, and $2\nu ECEC$, is presented. The calculation makes use of exact Dirac wave functions with finite nuclear size and electron screening and includes life-times, single and summed positron spectra, and angular positron correlations.

Sensitiveness of the ratio between monovacancy and bulk positron lifetimes to the approximations used in the calculations: Periodic behaviour

Positron lifetimes have been calculated in bulk and monovacancies for most of the elements of the periodic table. Self-consistent and non-self-consistent schemes have been used for the calculation of the electronic structure in the solid, as well as different parameterizations for the positron enhancement factor and correlation energy. The ratio between the monovacancy and bulk lifetimes has been analyzed. This ratio shows a periodic behaviour with atomic number in all the calculation methods and it is in agreement with selected experimental data. The ratio shows, in contradiction to previous assumptions, sensitiveness to the approximations used in the calculations. This extensive work has allowed us to study and enlighten features of the theory and computing methods broadly used nowadays in simulating, studying and understanding positronic parameters.

Bound states of positron with simple carbonyl and aldehyde species with configuration interaction multi-component molecular orbital and local vibrational approaches

Characteristic features of the positron-binding structure of some carbonyl and aldehyde species such as formaldehyde, acetaldehyde, acetone and propionaldehyde are discussed with the configuration interaction scheme of multi-component molecular orbital (MC_MO) calculations. This method can take the electron–positron correlation contribution into account through single electronic–single positronic excitation configurations. Our vertical positron affinity (PA) values of acetaldehyde and acetone with electronic 6-31 + +G(2df,2pd) and positronic [15s15p3d2f1g] basis sets are as 52 and 92 meV, which can be compared to the recent experimental values of 90 and 173 meV by Danielson et al (2010 Phys. Rev. Lett.104 233201). For formaldehyde we have also found that the PA values are enhanced by including the local vibrational contribution from the vertical PA value of 15 meV to 17, 21 and 25 meV after averaging over the zeroth, first and second vibrational states, respectively, due to the anharmonicity of the potential.

Calculation of positron characteristics for elements of the periodic table

Positron characteristics have been calculated in bulk and monovacancies for most of the elements of the periodic table. Self-consistent and non-self-consistent schemes have been used for the calculation of the electronic structure in the solid, and different parametrizations for the positron enhancement factor and correlation energy. As it is known, positron lifetimes in bulk show a periodic behaviour with atomic number. These calculations also confirm that monovacancy lifetimes follow the same behaviour. The results obtained have been compared with selected experimental lifetime data, which confirms the calculated theoretical trends. Positron binding energies to a monovacancy have been calculated also for most of the elements of the periodic table. The binding energy shows a periodic behaviour with atomic number too.

Bound states of the positron with nitrile species with a configuration interaction multi-component molecular orbital approach

Characteristic features of the positron binding structure of some nitrile (-CN functional group) species such as acetonitrile, cyanoacetylene, acrylonitrile, and propionitrile are discussed with the configuration interaction scheme of multi-component molecular orbital calculations. This method can take the electron-positron correlation contribution into account through single electronic-single positronic excitation configurations. Our PA value of acetonitrile with the electronic 6-31++G(2df,2pd) and positronic [15s15p3d2f1g] basis set is calculated as 4.96 mhartree, which agrees to within 25% with the recent experimental value of 6.6 mhartree by Danielson et al. [Phys. Rev. Lett., 2010, 104, 233201]. Our PA values of acrylonitrile and propionitrile (5.70 and 6.04 mhartree) are the largest among these species, which is consistent with the relatively large dipole moments of the latter two systems.

Effect of mono vacancy on electron and positron properties of 3C SiC

AbstractFirst principle calculation of the electron and positron band structure in the perfect and defected 3C silicon carbide (SiC) is performed within the linear muffin‐tin orbital (LMTO) method in the atomic spheres approximation (ASA). For Si and C vacancy the insulator‐metal transition is observed. Effect of the electron–positron interaction on the resulting positron distribution is studied. Theoretical electron and electron–positron momentum densities for Si and C mono vacancies, obtained within various approximations to the electron–positron correlations, are discussed in comparison with the relevant characteristics in the perfect 3C SiC.

Positron hydrogen molecule scattering under Ro‐vibrational LFCC scheme using PCOP potential below the electronic excitation threshold

AbstractRecently we have presented a Laboratory Frame Ro‐Vibrational Close Coupling (LFCC) calculation for positron hydrogen molecule scattering process using Positron Correlation Polarization (PCOP) Potential and discussed the results below positronium formation threshold (Mukherjee and Sarkar, J. Phys. B 41 125201 (2008) [1]). In the present article we have reported total (integrated) cross sections as well as vibrational excitation cross sections in higher positron energy region below the first electronic excitation threshold of hydrogen molecule for same system with same coupling scheme. The results are compared with the existing elaborate theories and the measured values. In the reported energy region the present total cross sections agree well with one of the measured values but differ with other. However, the present theoretical values for 0 → 1 and 0 → 2 vibrational excitation cross sections vary appreciably with the other results. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Oxidation and thermal reduction of the Cu(1 0 0) surface as studied using positron annihilation induced Auger electron spectroscopy (PAES)

Changes in the surface of an oxidized Cu(1 0 0) single crystal resulting from vacuum annealing have been investigated using positron annihilation induced Auger electron spectroscopy (PAES). PAES measurements show a large increase in the intensity of the annihilation induced Cu M 2,3VV Auger peak as the sample is subjected to a series of isochronal anneals in vacuum up to annealing temperature 300 °C. The intensity then decreases monotonically as the annealing temperature is increased to ∼600 °C. Experimental probabilities of annihilation of surface-trapped positrons with Cu 3p and O 1s core-level electrons are estimated from the measured intensities of the positron annihilation induced Cu M 2,3VV and O KLL Auger transitions. Experimental PAES results are analyzed by performing calculations of positron surface states and annihilation probabilities of surface-trapped positrons with relevant core electrons taking into account the charge redistribution at the surface, surface reconstructions, and electron–positron correlations effects. The effects of oxygen adsorption on localization of positron surface state wave function and annihilation characteristics are also analyzed. Possible explanation is proposed for the observed behavior of the intensity of positron annihilation induced Cu M 2,3VV and O KLL Auger peaks and probabilities of annihilation of surface-trapped positrons with Cu 3p and O 1s core-level electrons with changes of the annealing temperature.

Positronium annihilation in silica aerogel studied by a positron age-momentum correlation technique

A high-performance positron age-momentum correlation (AMOC) spectrometer was newly developed. The counting rate is increased up to 200 cps much larger than the value 20 cps reported by other international groups. And at the same time, the time resolution still keeps at the international level of 220 ps. Furthermore, positronium (Ps) annihilation in silica aerogel was investigated by AMOC, which indicates: (1) Ps annihilation between the grains dominantly undergoes pick-off process and spin conversion from o-Ps to p-Ps; (2) Annealing below 400 °C changes the grain surface conditions, i. e. the desorption of hydrogen and the decrease of the defect centers concentration.

The effect of the positron distribution and electron–positron correlations on theelectron–positron momentum density for SiC

To interpret the data on positron annihilation in solids in terms of the electron momentumdensity, both the electron–positron interaction and the positron distribution have to beconsidered explicitly. In this work we discuss the influence of the shape of the positronwavefunction and electron–positron (e–p) correlations on the calculated e–p momentumdensity and lifetime of the positron in perfect SiC. It is shown that the form of the positrondistribution in the Wigner–Seitz cell has a considerable effect on the calculated annihilationcharacteristics. The results obtained within the independent particle model are comparedwith their counterparts incorporating e–p correlation functions locally and beyond the localdensity approximation. The calculations have been performed for SiC of 3C cubic(zinc-blende) structure within the linear muffin-tin orbital atomic sphere approximationmethod.

Ro-vibrational close coupling study of positron–hydrogen molecule scattering using the parameter-free model correlation polarization potential

In this paper, we have presented the ro-vibrational laboratory frame close coupling (LFCC) calculation for the positron–hydrogen molecule scattering process. We have reported here (integrated) total cross section as well as vibrational and rotational excitation cross sections in the low-energy region of the incident positron. To calculate the scattering parameters we have used the positron correlation polarization potential (PCOP), which is especially meant for the positron as the incident particle. The present results are compared with the existing elaborate theories and the measured values. The present total cross sections agree well with the measured values, whereas the absolute values of the 0 → 1 vibrational excitation cross sections differ marginally. There are some differences between the present result and the other theoretical predictions. The reasons behind the differences are discussed.

Two-Component Density Functional Calculations of Positron Lifetimes for Band-Gap Crystals

Lifetimes of positrons in perfect crystals having band gaps are calculated using the two-component density functional theory. For electron–positron correlation, we use the local density approximation (LDA) and the correction of the LDA using a high-frequency dielectric constant. While the LDA is found to slightly underestimate positron lifetimes, the correction of the LDA well reproduces experimental lifetimes. We discuss the quality of crystals used in a positron annihilation experiment by comparing theoretical and experimental positron lifetimes.

Polarisation effects in low-energy positron–molecule scattering

The UK molecular R-matrix method has been adapted to treat positron collisions from polyatomic targets. A simple empirical enhancement factor which corrects for the underestimation of electron–positron polarisation and correlation effects in the calculations performed with the static-plus-polarization model at low scattering energies is presented. Application of this model to positron scattering from carbon dioxide at energies below 8eV is discussed. Introduction of the enhancement factor improves the integral cross sections significantly and introduces structures in the differential cross sections consistent with other studies. The prospects for a fully ab initio treatment of this problem are discussed.