Positron Affinity Research Articles

Theoretical study of a positron-attachment to vibrational excited states for non-polar carbon disulfide molecule

We theoretically analyzed a positron affinity (PA), which is the binding energy of a positron, of the non-polar carbon disulfide (CS2) molecule at vibrational excited states to elucidate the effect of molecular vibrations on the binding of positron to the molecule. Using the configuration interaction method of the multi-component molecular orbital theory and anharmonic vibrational state analysis with vibrational quantum Monte Carlo technique, the vibrational averaged PA values are calculated as 0.39, 2.03, and 5.02 meV for the ground state, fundamental tone, and overtone states of asymmetric stretching mode, respectively. The PA value of CS2 molecule is found to be enhanced by the vibrational excitation of only asymmetric stretching mode compared to the value at the vibrational ground state. With the linear regression analysis, we have confirmed that such enhancement of vibrational averaged PA values mainly arises from the increment of molecular permanent dipole moment due to the vibrational excitations of the asymmetric stretching mode.

Positron Characteristics in Cadmium and Zinc Chalcogenides

Electron energy levels and positron states have been calculated for cadmium and zinc chalcogenide compounds within the pseudo-potential approach and the independent particle model. Furthermore, the present contribution deals with the electron and positron chemical potentials allowing the calculation of the positron affinity to different materials of interest and hetero-structures formed by these materials. Besides, we here determine the positron diffusion constant by means of the positron deformation potential. An attempt has been made to scale positron affinity and diffusion constant with the lattice constant and the band gap energy, respectively. Such scaling is found to be not possible. The information gathered by the present study is of prime importance for a better understanding of positron trapping at interfaces and precipitates and should be useful in slow positron beam experiments.

Theoretical calculation of positron affinities of solute clusters in aluminum alloys

We have performed theoretical calculations of positron states for solute clusters in aluminum alloys to estimate the positron affinity of solute clusters. Positron states of solute clusters in aluminum alloys were calculated under the electronic structures obtained by first- principles molecular orbital calculations using Al158-X13 clusters. We defined the positron affinity of the solute clusters by the difference in the lowest potential sensed by positrons between the solute clusters and Al bulk. With increasing atomic number of 3d metals, the annihilation fraction of the solute clusters rapidly increases at Mn and shows a maximum at Ni. A similar trend is observed for 4d metals. The localization of positron at the solute clusters mainly arises from charge transfer from Al matrix to solute clusters. The positron affinity defined in this work well represents the localization of positron at the solute clusters in aluminum alloys.

Metal-Semiconductor Interfaces Investigated by Positron Annihilation Spectroscopy

Variable-energy positron annihilation spectroscopy has been applied to study interfaces in Al/Si, Au/Si and Au/GaAs structures. Computational fittings of ROYPROF program were used to analyze Doppler broadening results in order to determine kinds of regions that positrons were likely to sample. The interfaces were found acting as a capturing thin layer with negligible positrons stopped in them and their characteristics came only from positrons diffusing to these interfaces, the positron work function of these materials were taken into consideration. In all fittings, the interfaces are found to have 1 nm thickness and act as an absorbing sink for all thermal positrons diffusing towards them, and this indicates either the existence of open volume defects or a weakness of known theoretical models for positron affinities. The result is supported by measurements obtained by applying external electric fields on Al/Si sample. Theoretical fittings have clearly demonstrated the sensitivity of interfaces in these attempts and their importance in data analyzing and in developing of fitting cods.

Positron annihilation and tribological studies of nano-embedded Al alloys

Positron annihilation studies of aluminium alloys with nanodispersions of insoluble elements, i.e., In, Sn, Pb and Au were reported. The alloys were obtained using a rapid solidification process. For all alloys, except that with Au, the average diameter of nanoparticles in aluminium matrix was 100 nm, and variance of the size distribution was above 50 nm. Positron annihilation studies reveal the presence of monovacancies or divacancies, which were located at the interface between nanoparticles and the matrix. In the as-cast reference pure aluminium sample as well as the aluminium and gold alloy dislocations were identified as well. The isothermal annealing of the obtained alloys and measurement of the annihilation characteristic, i.e., S-parameter, allow us to determine the activation energy of grain boundary migration, which for the alloys was higher by the factor of four than for the reference sample. The measurements of friction parameters for the alloys confirmed the results reported by the other authors that, the friction coefficient was lower by the factor of about two and the specific wear rate was by the factor of about fifty higher than the reference sample. The present study confirmed the attractive positron affinity of the nanoparticles of In, Sn, Pb and Au compared to aluminium matrix.

Exploring positron characteristics utilizing two new positron–electron correlation schemes based on multiple electronic structure calculation methods**Project supported by the National Natural Science Foundation of China (Grant Nos. 11175171 and 11105139).

We make a gradient correction to a new local density approximation form of positron–electron correlation. The positron lifetimes and affinities are then probed by using these two approximation forms based on three electronic-structure calculation methods, including the full-potential linearized augmented plane wave (FLAPW) plus local orbitals approach, the atomic superposition (ATSUP) approach, and the projector augmented wave (PAW) approach. The differences between calculated lifetimes using the FLAPW and ATSUP methods are clearly interpreted in the view of positron and electron transfers. We further find that a well-implemented PAW method can give near-perfect agreement on both the positron lifetimes and affinities with the FLAPW method, and the competitiveness of the ATSUP method against the FLAPW/PAW method is reduced within the best calculations. By comparing with the experimental data, the new introduced gradient corrected correlation form is proved to be competitive for positron lifetime and affinity calculations.

Theoretical analysis of positron-attachment to small molecules: Vibrational enhancement of positron affinities

SynopsisTo theoretically demonstrate the binding of a positron to small polarized molecules, we have calculated the vibrational averaged positron affinity (PA) values for small molecules with multi-component quantum Monte Carlo and molecular orbital methods. Our most accurate prediction of the vibrational averaged PA values at the fundamental and overtone states for formaldehyde molecule are 31 and 36 meV, respectively.

Early stage ageing effects and shallow positron traps in Al–Mg–Si alloys

Room temperature ageing, so-called natural ageing, of Al–Mg–Si alloys has a subtle but striking influence on the mechanical properties achievable by subsequent ageing at more elevated temperatures. Though strongly debated, different clustering processes are generally accepted to give rise to this effect. Using temperature-dependent positron lifetime measurements of naturally aged Al–Mg–Si alloys, it is shown that in the early stages of ageing, small clusters of alloying atoms without embedded vacancies take part in the decomposition process. These clusters serve as shallow positron traps with a binding energy of about 55(10) meV, grow in the course of natural ageing and transform to deep positron traps with binding energies well above thermal energies. Thus, results of positron annihilation spectroscopy techniques need to be interpreted carefully with respect to the microstructure of age-hardenable Al alloys. Moreover, it is shown that a simple approach to bind positron states using a three-dimensional potential well and (bulk) positron affinities cannot explain the present findings.

Proposed parameter-free model for interpreting the measured positron annihilation spectra of materials using a generalized gradient approximation.

Positron annihilation spectroscopy is often used to analyze the local electronic structure of materials of technological interest. Reliable theoretical tools are crucial to interpret the measured spectra. Here, we propose a parameter-free gradient correction scheme for a local-density approximation obtained from high-quality quantum MonteCarlo data. The results of our calculations compare favorably with positron affinity and lifetime measurements, opening new avenues for highly precise and advanced positron characterization of materials.

Positron accumulation effect in particles embedded in a low-density matrix

Systematic studies of the so-called positron accumulation effect for samples with particles embedded in a matrix are reported. This effect is related to energetic positrons which penetrate inhomogeneous medium. Due to differences in the linear absorption coefficient, different amounts of positrons are accumulated and annihilate in the identical volume of both materials. Positron lifetime spectroscopy and Doppler broadening of the annihilation line using Na-22 positrons were applied to the studies of the epoxy resin samples with embedded micro-sized particles of transition metals, i.e., Ni, Sn, Mo, W, and nonmetal particles, i.e., Si and NaF. The significant difference between the determined fraction of positrons annihilating in the particles and the particle volume fraction indicates the positron accumulation effect. The simple phenomenological model and Monte Carlo simulations are able to describe the main features of the obtained dependencies. The aluminum alloy with embedded Sn nanoparticles is also considered for demonstration differences between the accumulation and another related effect, i.e., the positron affinity.

Theoretical prediction of the binding of a positron to a formaldehyde molecule using a first-principles calculation

The vertical positron affinity (PA) value at the equilibrium position of a formaldehyde molecule is predicted as +25(3) meV with the highly accurate quantum Monte Carlo method. Applying anharmonic vibrational analysis, we have found that the vibrational excitation of the C = O stretching mode drastically enhances the PA value, due to the increment of the molecular dipole moment along this mode. Our predictions of the vibrational averaged PA values at the fundamental and overtone states are 31 and 36 meV, respectively, which strongly supports the conclusion that a positron can bind to formaldehyde.

Evidence of zirconium nano-agglomeration in as-cast dilute U–Zr alloys

Microstructure evaluation of as-cast and annealed U–Zr (Zr=2, 6 and 10wt.%) alloys has been carried out for the first time using positrons as a probe. The chemical signature in the matter–antimatter annihilation gamma and the positron lifetime data suggests that majority of positrons are annihilating from Zr sites in the as-cast alloys. The results have been interpreted as due to the presence of Zr nano-agglomerates in the as-cast alloys which have a higher positron affinity as compared to the rest of the U matrix. A minimum agglomerate size of ∼2nm diameter has been calculated from the difference in positron affinity between the agglomerates and the matrix. Upon annealing, the Zr signature in the annihilation gamma photons vanishes suggesting that the Zr agglomerates diffuse out of U matrix and form micron-sized precipitates. This has been confirmed by scanning electron microscopy which shows a 3 times increase in the surface density of the precipitates in the annealed alloys as compared to the as-cast ones. Shorter positron diffusion length (measured using slow positron beam) as compared to precipitate separation has been invoked to explain the observed data.

Theoretical investigation of the binding of a positron to vibrational excited states of hydrogen cyanide molecule

We theoretically analyzed positron affinities (PA) of hydrogen cyanide (HCN) molecule at vibrational excited states to elucidate the effect of molecular vibrations on the binding of a positron to the molecule. Using the configuration interaction method in the multi-component molecular orbital theory and anharmonic vibrational state analysis with the variational Monte Carlo technique, we found that the vibrational excitations of the CN and CH stretching modes enhance the PA value compared to that of the vibrational ground state, whereas the excitation of bending mode deenhances it. The largest PA enhancement is found at the excited states of the CH stretching mode; the PA values are 43.02 (1) and 46.34 (2) meV for the fundamental tone and overtone states, respectively. With the linear regression analysis, we confirmed that the PA variation of HCN molecule at each vibrational state arises from the variation of permanent dipole moment and dipole-polarizability due to each vibrational excitation.

Positron-attachment to acetonitrile, acetaldehyde, and acetone molecules: Vibrational enhancement of positron affinities with configuration interaction level of multi-component molecular orbital approach

To theoretically demonstrate the binding of a positron to acetonitrile, acetaldehyde, and acetone molecules, we have calculated the vibrational averaged positron affinity (PA) values along the local CN or CO vibrational contribution with the configuration interaction level of multi-component molecular orbital method. This method can take the electron-positron correlation contribution into account through single electronic – single positronic excitation configurations. The PA values are enhanced by including the local vibrational contribution from vertical PA values due to the anharmonicity of the potential. For acetonitrile, acetaldehyde, and acetone molecules, the PA values after averaging over the 1st vibrational state are 136, 55, and 96 meV, which can be compared with the corresponding experimental vlues of 180, 90, and 173 meV, respectively.

Improved generalized gradient approximation for positron states in solids

Several first-principles calculations of positron-annihilation characteristics in solids have added gradient corrections to the local-density approximation within the theory by Arponen and Pajanne [Ann. Phys. (N.Y.) 121, 343 (1979)] since this theory systematically overestimates the annihilation rates. As a further remedy we propose to use gradient corrections for other local density approximation schemes based on perturbed hypernetted-chain and on Quantum Monte Carlo results. Our calculations for various metals and semiconductors show that the proposed schemes generally improve the positron lifetimes when they are confronted with experiment. We also compare the resulting positron affinities in solids with data from slow-positron measurements.

Theoretical investigation of a positron binding to an aspartame molecule using the ab initio multicomponent molecular orbital approach

The feature of positron binding to aspartame molecule was analyzed using the ab initio multicomponent molecular orbital method. All nine stable conformers for aspartame molecule have positive positron affinity (the binding energy of a positron) values, which means that a positron can be attached to parent aspartame molecule. Analyzing the positronic orbitals of positronic aspartame conformers and the electrostatic potential maps of the corresponding parent molecules, we found that long‐range electrostatic interaction is the most crucial role in positron binding to aspartame molecule. We theoretically confirmed the possibility of positron binding to the conformers of aspartame molecule with strong dipole moment. © 2014 Wiley Periodicals, Inc.

Corrigendum: Vibrational enhancement of positron affinities for nonpolar carbon dioxide and carbon disulfide molecules: Multi-component molecular orbital study for vibrational excited states

To demonstrate the binding of a positron to nonpolar CX2 (X = O and S) molecules, we have estimated the vibrational averaged positron affinity (PA) values for the harmonic asymmetric stretching vibrational coordinate with the multi-component molecular orbital method. A positron can be attached at higher vibrational excitation levels, and vibrational averaged PA values become greater as the number of vibrational quantum number increases. The resultant PA values of carbon disulfide (CS2) molecule are greater than those of CO2, which is consistent with the corresponding property such as infrared (IR) intensity and polarizability of the parent species. © 2012 Wiley Periodicals, Inc.

Positron-attachment to nonpolar or small dipole CXY (X, Y = O, S, and Se) molecules: vibrational enhancement of positron affinities with configuration interaction level of multi-component molecular orbital approach

To theoretically demonstrate the binding of a positron to a nonpolar or small dipole molecule, we have calculated the vibrational averaged positron affinity (PA) values along the harmonic asymmetric stretching vibrational coordinate with the configuration interaction level of multi-component molecular orbital method for CXY (X, Y = O, S, and Se) molecules. For CSe2 and CSSe molecules, a positron can even be attached at the equilibrium structures, due to the effect of the induced dipole moment with large polarizability values. For a CS2 molecule, the positive PA value is obtained at the lowest vibrational excited state in our scheme. Although there is no direct experimental evidence for the positron-binding to CO2, COS, and COSe molecules, we have predicted positron-binding for these molecules at higher vibrational excited states.

Systematic theoretical investigation of a positron binding to amino acid molecules using the ab initio multi-component molecular orbital approach

The feature of positron binding to twenty neutral amino acid molecules with the global minimum (GM) and intramoleculear hydrogen-bonded (HB) structures was analysed using the ab initio multi-component molecular orbital method. All amino acid molecules in the intramoleculear HB structures have positive positron affinity (PA, the binding energy of a positron) values, which means that a positron can be attached to parent molecules, while in the GM structures only three amino acid molecules of Gln, Trp, and His have positive PA values. Analysing the positronic orbitals of each positronic amino acid molecule and the electrostatic potential maps of the corresponding parent molecules, we found that long-range electrostatic interaction is the most crucial role in positron binding to neutral amino acid molecules. The strong correlation is observed between the PA and dipole moment, that is, a polar molecule with a large dipole moment has a large PA value. The critical dipole moment of neutral amino acid molecules for binding a positron was found at about 3.46 Debye. From the systematic analysis of twenty kinds of amino acid molecule, we theoretically confirmed the possibility of positron binding to conformers of amino acid molecules with strong dipole moment.

Ab initio study of the positronation of the CaO and SrO molecules including calculation of annihilation rates

Ab initio multireference single- and double-excitation configuration interaction calculations have been performed to compute potential curves for ground and excited states of the CaO and SrO molecules and their positronic complexes, e(+)CaO, and e(+)SrO. The adiabatic dissociation limit for the (2)Σ(+) lowest states of the latter systems consists of the positive metal ion ground state (M(+)) and the OPs complex (e(+)O(-)), although the lowest energy limit is thought to be e(+)M + O. Good agreement is found between the calculated and experimental spectroscopic constants for the neutral diatomics wherever available. The positron affinity of the closed-shell X (1)Σ(+) ground states of both systems is found to lie in the 0.16-0.19 eV range, less than half the corresponding values for the lighter members of the alkaline earth monoxide series, BeO and MgO. Annihilation rates (ARs) have been calculated for all four positronated systems for the first time. The variation with bond distance is generally similar to what has been found earlier for the alkali monoxide series of positronic complexes, falling off gradually from the OPs AR value at their respective dissociation limits. The e(+)SrO system shows some exceptional behavior, however, with its AR value reaching a minimum at a relatively large bond distance and then rising to more than twice the OPs value close to its equilibrium distance.