Gamma-ray spectra resulting from the annihilation of positrons with selected core levels of Cu, Ag, and Au

Kim, S.; Eshed, Anat; Goktepeli, S.; Sterne, P. A.; Köymen, A. R.; Chen, W. C.; Weiss, A. H.

doi:10.1103/physrevb.73.014114

“…parabolic in the semilogarithmic plots). If one assumes that both the electron momentum and the true annihilation momentum atomic densities are Gaussians, then then the atomic correction factor G(P), (24), will also be a Gaussian. It is easy to see from equations (5) and (10) that in this case the corresponding annihilation spectra are also Gaussian.…”

Section: Gamma Spectra With Atomic Correction Factorsmentioning

confidence: 99%

“…In light of this fact, positron annihilation spectroscopy is a valuable technique in the study of surfaces, interfaces and defects in condensed matter [10,11], of electronic properties of quantum dots [12][13][14][15][16][17], nanoparticles [18,19] and nanocrystals [20], and in the study and exploitation of positron annihilation on core electrons [4,8,[21][22][23][24][25].…”

Section: Introductionmentioning

confidence: 99%

Effect of positron–atom interactions on the annihilation gamma spectra of molecules

Green

¹

,

Saha

²

,

Wang

³

et al. 2012

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Calculations of γ -spectra for positron annihilation on a selection of molecules, including methane and its fluoro-substitutes, ethane, propane, butane and benzene are presented. The annihilation γ -spectra characterise the momentum distribution of the electron-positron pair at the instant of annihilation. The contribution to the γ -spectra from individual molecular orbitals is obtained from electron momentum densities calculated using modern computational quantum chemistry density functional theory tools. The calculation, in its simplest form, effectively treats the low-energy (thermalised, room-temperature) positron as a plane wave and gives annihilation γ -spectra that are about 40% broader than experiment, although the main chemical trends are reproduced. We show that this effective 'narrowing' of the experimental spectra is due to the action of the molecular potential on the positron, chiefly, due to the positron repulsion from the nuclei. It leads to a suppression of the contribution of small positron-nuclear separations where the electron momentum is large. To investigate the effect of the nuclear repulsion, as well as that of short-range electron-positron and positron-molecule correlations, a linear combination of atomic orbital description of the molecular orbitals is employed. It facilitates the incorporation of correction factors which can be calculated from atomic manybody theory and account for the repulsion and correlations. Their inclusion in the 4 2 calculation gives γ -spectrum linewidths that are in much better agreement with experiment. Furthermore, it is shown that the effective distortion of the electron momentum density, when it is observed through positron annihilation γ -spectra, can be approximated by a relatively simple scaling factor. annihilation on a range of molecules including hydrogen, methane and its fluoro-substitutes, ethane, propane, butane and benzene.The first measurement of the Doppler-broadened γ -spectrum for positron annihilation on molecules was made in 1986 by Brown and Leventhal in low-density hydrogen gas [27]. In the early 1990s the confinement of thermalized positrons in a Penning trap allowed Tang et al [28] to measure γ -spectra for a range of molecules, including hydrocarbons and perfluorocarbons. This work was built upon by Iwata et al [3] who measured γ -spectra for positron annihilation on a large range of atoms and molecules in the gas phase. Despite this extensive set of experimental results, there is a paucity of theoretical calculations of the annihilation γ -spectra in molecules. Theoretical predictions of the γ -spectra do exist for the molecular ion H + 2 , for which (numerical) Coulomb-Born and configuration-interaction Kohn variational calculations have been performed [29], and for the small molecules H 2 and N 2 [30,31]. However, the application of these sophisticated methods to larger molecules is considerably more difficult. More than thirty years ago Chuang and Hogg [32] analysed the annihilation γ -spectra for hexane and decane. However, their ca...

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“…Positron annihilation on core electrons is also a key process in the surface-analytic positroninduced Auger-electron spectroscopy (PAES) [7][8][9][10], and the time-resolved PAES [11], which enables the study of dynamics of catalysis, corrosion, and surface alloying [12]. Coincident measurements of the annihilation γ-ray and Auger electrons allows one to determine the annihilation γ-ray spectra for individual core orbitals [13,14].…”

mentioning

confidence: 99%

γ-Ray Spectra and Enhancement Factors for Positron Annihilation with Core Electrons

Green

¹

,

Gribakin

²

2015

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Many-body theory is developed to calculate the γ-spectra for positron annihilation with valence and core electrons in the noble gas atoms. A proper inclusion of correlation effects and core annihilation provides for an accurate description of the measured spectra [Iwata et al., Phys. Rev. Lett. 79, 39 (1997)]. The theory enables us to calculate the enhancement factors γ nl , which describe the effect of electron-positron correlations for annihilation on individual electron orbitals nl. We find that the enhancement factors scale with the orbital ionization energy I nl (in electron-volt), as γ nl = 1 + A/I nl + (B/I nl ) β , where A ≈ 40 eV, B ≈ 24 eV and β ≈ 2.3.

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“…The method is suggested by an observation that the momentum densities and the γ-spectra in figures 7 and 8 are approximately Gaussian (i.e., parabolic in the semilogarithmic plots). If one assumes that both the electron momentum and the true annihilation momentum atomic densities are Gaussians, then then the atomic correction factor G(P ), (24), will also be a Gaussian. It is easy to see from Eqs.…”

Section: Gamma Spectra With Atomic Correction Factorsmentioning

confidence: 99%

“…In light of this fact, positron annihilation spectroscopy is a valuable technique in the study of surfaces, interfaces and defects in condensed matter [10,11], of electronic properties of quantum dots [12][13][14][15][16][17], nanoparticles [18,19] and nanocrystals [20], and in the study and exploitation of positron annihilation on core electrons [4,8,[21][22][23][24][25].…”

Section: Introductionmentioning

confidence: 99%

Effect of positron-atom interactions on the annihilation gamma spectra of molecules

Green,

Saha,

Wang

et al. 2012

Preprint

0

View full text Add to dashboard Cite

Calculations of γ-spectra for positron annihilation on a selection of molecules, including methane and its fluoro-substitutes, ethane, propane, butane and benzene are presented. The annihilation γ-spectra characterise the momentum distribution of the electron-positron pair at the instant of annihilation.The contribution to the γ-spectra from individual molecular orbitals is obtained from electron momentum densities calculated using modern computational quantum chemistry density functional theory tools. The calculation, in its simplest form, effectively treats the low-energy (thermalised, room-temperature) positron as a plane wave and gives annihilation γ-spectra that are about 40% broader than experiment, although the main chemical trends are reproduced. We show that this effective "narrowing" of the experimental spectra is due to the action of the molecular potential on the positron, chiefly, due to the positron repulsion from the nuclei. It leads to a suppression of the contribution of small positron-nuclear separations where the electron momentum is large. To investigate the effect of the nuclear repulsion, as well as that of short-range electron-positron and positron-molecule correlations, a linear combination of atomic orbital description of the molecular orbitals is employed. It facilitates the incorporation of correction factors which can be calculated from atomic many-body theory and account for the repulsion and correlations. Their inclusion in the calculation gives γ-spectrum linewidths that are in much better agreement with experiment. Furthermore, it is shown that the effective distortion of the electron momentum density, when it is observed through positron annihilation γ-spectra, can be approximated by a relatively simple scaling factor.

show abstract

Gamma-ray spectra resulting from the annihilation of positrons with selected core levels of Cu, Ag, and Au

Cited by 9 publications

References 23 publications

Effect of positron–atom interactions on the annihilation gamma spectra of molecules

Effect of positron–atom interactions on the annihilation gamma spectra of molecules

γ-Ray Spectra and Enhancement Factors for Positron Annihilation with Core Electrons

Effect of positron-atom interactions on the annihilation gamma spectra of molecules

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