New evaluation of the angular correlation of positron annihilation spectra for noble and d-transition metals

A new fit to the long-range angular correlation of annihilation radiation data of the sum of a parabola, so-called Ferrell function, and a Gaussian for noble and d-transition metals is presented. The three functions are considered to describe the positron annihilation with three different electron groups: nearly free, d-like and rare-gas core electrons, respectively.     

Momentum distribution in benzene: A comparison of compton scattering and positron annihilation

Probabilities in momentum space for the benzene molecule are calculated for two cases: An ordinary benzene molecule, and benzene with a bound positron which is coalesced with one of the electrons. The former probability is related to the Compton scattering profile; the latter, to angular correlation measurements made in positron annihilation experiments. In this work we make two comparisons on the basis of quantum mechanical calculations; a) between Compton scattering and positron annihilation results, and b) among the several possible positron annihilation results associated with different symmetries for the positronic molecular orbital (PMO). The Compton scattering results are found to be similar to the positron annihilation results for the more symmetrical PMOs; and all these are quite different from the positron annihilation results for the less symmetrical PMOs. A suggestion for a crucial experiment is made.     

Properties of positron annihilation in metals

Positron annihilation in bulk metals has been studied by examining the angular distribution of the annihilation photons in polycrystalline samples of magnesium, aluminum, copper, zinc, indium, tin, lead, and bismuth. It has been shown that conduction electrons as well as core electrons take part in this process. The conduction electron densities and Fermi energies have been determined. It is found that the electron density in the vicinity of a positron is significantly higher than the density of the free electron gas. We believe that this is due to the formation of Wheeler complexes and we estimate its charge. We have analyzed various means of measuring the conduction electron density and conclude that the positron method gives the most reliable information. Fiz. Tverd. Tela (St. Petersburg) 41, 929–934 (June 1999)     

Momentum density measurements by positron annihilation in metals and alloys; Recent experiments with a multicounter two-dimensional angular correlation apparatus

The technique of positron annihilation as applied to the study of momentum densities and Fermi surfaces is reviewed. The angular correlation of the two annihilation photons is directly related to the momentum distribution of the positron-electron system; breaks in this distribution reveal the size and shape of the Fermi surface. After a general introduction to the theory and the experimental techniques used, the results yielding various features of the fermi surface in high concentration disordered alloys are reviewed, and are compared with theoretical predictions. A new multicounter two-dimensional correlation apparatus is described and results in several solids are presented.     

Two-dimensional electron momentum distribution in graphite revealed by means of angular correlation of positron annihilation

Two-dimensional angular correlation measurements with positron annihilation radiation (2D-ACAR) on a single crystal of Kish graphite reveal that the motion of the annihilating electrons is nearly free along the graphite layers and restricted within some distance shorter than the lattice constant in the direction perpendicular to the layers. The hollows appearing on the 2D-ACAR surfaces are explained in terms of preferential annihilation of positrons with π-electrons in graphite.     

Doppler broadening of positron annihilation photons in metals

The Doppler broadening of positron annihilation photons was measured in 17 metals. A model which considers the positron lifetime spectra in metals to be composed of terms for annihilation with conduction and core electrons and surface centers of low electron momentum is used to correlate calculated core annihilation rates with the Doppler lineshape. Ta metal was doped with defects with high energy implantations of¹⁴N⁺⁴ ions at variable doses. Differences in the Doppler linewidths were ascribed as being principally a reflection of the probabilty of annihilation with core electrons relative to annihilation with conduction electrons.     

Magnetic field effects on positron annihilation in metals

It is shown that the positron annihilation rate from a state with momentum k parallel to the magnetic field is periodic in $\text{1}\text{B}$, and that the periodicity is determined by the area of the section of the fermi surface whose z component of momentum is k.     

Multichannel scattering and annihilation in the positron hydrogen system

A problem to account for the direct electron-positron annihilation in positron-hydrogen scattering above the positronium formation threshold has been resolved within the time independent formalism. On the basis of the generalized optical theorem, the annihilation cross section is fully determined by scattering amplitudes. This allows to perform the analytic separation of the contribution of the positronium formation effect from the overall annihilation cross section. The rest determines the direct annihilation cross section.     

Deconvolution of Doppler-broadened positron annihilation energy spectra in metals

The development of computer programmes to deconvolute and fit the Dopplerbroadened annihilation radiation from a moving positron-electron pair is discussed. An analysis is presented for the annihilation line observed in a series of metals using a high-resolution Ge(Li) detector.     

Measurement of the angular correlation of positron annihilation radiation using a Ge(Li) detector

The use of a Ge(Li) detector as one of the radiation detectors in a long slit angular correlation measurement of the annihilation radiation is described. The measurement yields information similar to that obtained by point slit or spark chamber geometry. The results for a copper single crystal are reported.     

Anisotropy in the angular correlation curves of NiO

The positron angular correlation curves were measured along the [100], [110] and [111] directions of NiO single crystals. A simple calculation based on the localized ion model shows that the measured anisotropy is ascribed to the uncompleted 3d orbitals of Ni with a slight covalent mixing of oxygen 2p orbitals.     

Angular correlation of two-photon positron annihilation in molecular hydrogen and nitrogen

The calculation of the angular correlation of two-photon annihilation radiation for thermalized positrons colliding with simple molecules, particularly molecular hydrogen and nitrogen, is discussed using a one-state approximation with polarization potentials. The results are compared with measurements in liquid H₂ and N₂ as well as with recent annihilation gammaray spectral measurements in gaseous targets. The agreement is good in the case of hydrogen but for nitrogen the calculated distribution is found to be substantially narrower than the measured values.     

The anisotropy of conduction electron scattering in the noble metals

The main purpose of this paper is to bring together experimental results which have yielded some quantitative information with respect to the anisotropy of conduction electron scattering in the noble metals. It is found that from the ordinary transport properties, and in particular from analyses of the Hall effect and deviations from Mattheissen's rule, the assembled conclusions do not present a coherent picture, at least with regard to electron-impurity scattering. The results for scattering by phonons are more consistent. The advent recently of rather precise (～ 0·1%) specifications of the noble metal Fermi surfaces together with the knowledge of certain derivative properties over them makes possible more direct measurements of electron scattering. The application of experimental techniques which are more commonly associated with the field of ‘Fermiology’, and in particular the de Haas-van Alphen effect, to study conduction electron scattering more directly is an important theme of the present paper. Such direct experiments, which can yield local values of the electronic lifetime, τ (k), make possible a comparison between theory and experiment prior to the stage where the Boltzmann equation or its equivalent needs to be solved.     

Phase transformations and vacancy formation energies of transition metals by positron annihilation

The technique of the coincidence count rate at the peak of the angular correlation curve (CCR) in positron annihilation has been applied to the investigation of vacancy formation energies in thermal equilibrium in nickel, cobalt, and iron. The monovacancy formation energyE ^1v/F has been determined to (1.55±0.05) eV and (1.34±0.07) eV for nickel and cobalt, and (1.60±0.10) eV for α-iron, and (1.40±0.15) eV for γ-iron, respectively. The structural phase transformations in cobalt (693 K) and iron (1183 K, 1663 K) are exhibited by discontinuities of the CCR. In the case of cobalt the CCR follows exactly the change of the thermal expansion at the transition temperature. The temperature dependence of the CCR in the prevacancy region is found to be proportional to the thermal expansion for all metals investigated.     

Effective concentration of electrons in metals upon measurements via positron annihilation spectroscopy

Calculations are performed together with the summarization of published data on the measurement of positron annihilation rates in a number of simple substances. The information potential of different methods underlying positron annihilation spectroscopy is analyzed as applied to the study of condensed matter. The features of the mechanism of positron annihilation in metals are discussed. The possibility of investigating the electronic and defect structures of metals and alloys using the method of the time distribution of annihilation photons is considered.     

Investigation of d-orbital participation in bonding by positron angular correlation

Whether or not ligand perturbation is necessary for d-orbital contraction and consequent participation in bonding has been controversial till now. A study of angular correlation of positron annihilation radiation on organic sulphides and sulphones gives a direct evidence for the conclusion that ligand perturbation is necessary.     

Study of C centres in NaCl by positron angular correlation

The nature of C centres in NaCl has been experimentally studied by the positron annihilation technique. The results are in support of a previously proposed model for the C centres [1]. The approximate size of C centres has been estimated to be of the order 6.3 ± 0.7 Å.     

Different ways of dealing with Compton scattering and positron annihilation experimental data

Different ways of dealing with one-dimensional (1D) spectra, measured e.g., in the Compton scattering or angular correlation of positron annihilation radiation (ACAR) experiments, are presented. Using the example of divalent hexagonal close packed metals, we show what kind of information on the electronic structure one can get from 1D profiles interpreted in terms of either 2D or 3D momentum densities.2D and 3D densities are reconstructed from merely two and seven 1D profiles, respectively. Applied reconstruction techniques are particular solutions of the Radon transform in terms of orthogonal Gegenabauer polynomials. We propose their modification connected with so-called two-step reconstruction.The analysis is performed both in the extended p and reduced k zone schemes. It is demonstrated that if the positron wave function or many-body effects are strongly momentum dependent, analysis of 2D densities folded into k space may lead to wrong conclusions concerning the Fermi surface. In the case of 2D ACAR data in Mg, we found very strong many-body effects. PACS 71.18.+y; 13.60.Fz; 87.59.Fm