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.     

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.     

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)     

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.     

Systematics in the core level spectra of Th-intermetallics

We have studied the Th 4f core level spectra in a number of related Th-intemetallics, which exhibit interesting trends in terms of the intensity and the energy separation of the satellite with respect to the main signal. With the help of model calculations, we correlate the changes in the core level spectra to the density of states (DOS) in the valence band region. We have studied in detail the effect of changes in the strenght of 5f-conduction band hybridization (), the energy position of the empty 5f level ( _f ) and the DOS in the vicinity ofE _F on the spectral shape of Th 4f region. These parameters are also found to affect the binding energies of the main peaks.     

TheLMM Auger spectra of some transition metals of the first series

TheLMM peaks in the Auger spectra for the first transition series are identified according to all the evidence currently available, with special reference to Fe, Co and Ni. The similarity of results obtained with different types of analyser is demonstrated.     

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.     

The effect of the core hole on x-ray emission spectra in simple metals

We show that one-particle calculations of x-ray spectra with and without the core hole can give drastically different results, indicating a breakdown of one-particle theory. Only emission spectra obtained in the absence of the core hole consistently show a close resemblance to experiment. We show, that this fact can be explained by the many-body theory of Nozières and DeDominicis. We are then able to give the first interpretation of the L_2,3 satellite spectrum of sodium.     

Calculated core hole effects on the K X-ray spectra of BCC transition metals

Self-consistent density functional calculations for the empty electronic states in V, Cr, Nb, Mo are used to assess the core hole effects on the K X-ray absorption near edge spectra. Initial and final state approximations with no and complete conduction electron screening of the hole are considered as limiting cases. The comparison shows that hole effects cannot explain the observed discrepancies between experiment and single particle calculations.     

Electronic structure and positron annihilation in alkali metals: Isolation of ionic core contribution and valence high-momentum components

Momentum densities of annihilation pairs from valence as well as from ionic core electrons in alkali metals are calculated ab initio and compared with the experimental results. It is shown that the valence high-momentum components constitute a great deal (23–34% in Na-Cs and probably even more in Li) of the Gaussian part of the angular correlation curves. The average core enhancement factor γ_c ranges from 1.5 (Li) to 7.1 (Cs) and may be well expressed by a logarithmic function of ionic core polarizability. The presented values of γ_c are much higher than the core enhancement factors in the high-momentum (?15 mrad) region which, according to the recent theory of Bonderup, Andersen and Lowy, should not be very different from unity.     

A simple approach to the analysis of positron lifetime spectra

A new method is proposed to deal with analysis of positron annihilation lifetime spectra which defy ordinary computer fitting. The method is particularly suited for investigations of defects in metals.     

Systematics of Mossbauer isomer shifts in transition metals

Mössbauer isomer shifts for ⁹⁹Ru, ¹⁹³Ir, and ¹⁹⁷Au as impurities in other transition metals are reported. Together with published data on ⁵⁷Fe, ¹⁸¹Ta, and ¹⁹⁵Pt, these results reveal a systematic behavior of the electron densities at the nuclei of the impurities. Some of these features can be useful for the derivation of values for σ〈r²〉 for new Mössbauer resonances.     

Plasmon coupling to core holes in simple metals

Soft X-ray appearance potential data for the metals lithium and beryllium are presented. The plasmon satellite structure is compared in both cases with recently available XP-spectra. For s-metals in general, the plasmon coupling constant in APS seems to be smaller than in the corresponding XPS experiment. Assuming the interaction to be primarily intrinsic, one would expect the coupling constants to be identical.     

Error sources for the analysis of Doppler broadening in positron annihilation spectra

Correction factors for systematic shifts of the second-order moment ² (linearly related toS ^–1) of the Doppler broadened annihilation gamma ray lineshape are presented. They allow to compare ² values obtained under different experimental conditions. The height of the observed peak, the number of keV/channels used and the number of channels over which the Doppler broadening dsstribution should be considered are bounded by the a priori settled relative uncertainty ofgs ².Fellow of the CIC, PciA Bs. As., Argentina     

Systematics of core line asymmetries in XPS spectra of Ni

Core line asymmetries have been observed in the 3p_{$\text{1}\text{2}$}, 3p_{$\text{3}\text{2}$}, 3s, 2p_{$\text{1}\text{2}$} and 2p_{$\text{3}\text{2}$} XPS spectra of Ni. The asymmetry parameters are the same for all lines within the experimental accuracy. The absence of exchange splitting in the 3s line indicates a 3d¹⁰ configuration for the fully screened final state of Ni.     

Electronic contributions to shear constants and susceptibility in cubic transition metals

We have calculated the electronic contributions to the shear constants C = c₄₄ and C' = (c₁₁ ? c₁₂)/2 of cubic transition metals, tight binding Hamiltonians and their derivatives with respect to shear, and thermodynamical perturbation theory. The results obtained for bare susceptibility and temperature derivatives of the shear constants on the basis of a band structure proposed by Posternak and Steinemann are in reasonable agreement with experiment.     

Parameters used for Doppler broadening spectra of positron annihilation γ-rays

The relationship between some popularly used arbitrary parameters and the variance of the Doppler broadening spectra of positron annihilation γ-rays is discussed. The experimental results are found to agree well with the theoretically derived formulas.