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Title: Positron annihilation on core electrons
Author: Green, D. G.
ISNI:       0000 0004 2724 7724
Awarding Body: Queen's University Belfast
Current Institution: Queen's University Belfast
Date of Award: 2012
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Diagrammatic many-body theory (MBT) is used to calculate the -y-spectra and an- nihilation rates for positrons annihilating on the core electrons of many-electron atoms. Long-range positron-atom and short-range positron-electron correlations are accounted for through the evaluation of the positron Dyson orbital and the true many-body annihilation vertex, which includes the exact electron-positron ladder series. The numerical implementation of the theory proceeds through the employment of a B-spline basis. It is tested through a comprehensive study of positron annihilation in the hydrogen-like ions He ", Li2+, BH and F8+, the ionization energies of which span those typical of the core electrons of many-electron atoms. The scattering phase shifts and normalized annihilation rate parameters Zeff are found to be in excellent agreement with existing sophisticated variational calculations, and -y-spectra are also predicted. , The annihilation ')'-spectra and partial annihilation rates are then calculated for ther- malized positrons annihilating on the core and valence electrons of the noble gas atoms Ar, Kr and Xe. Although stronger for the valence shells, the short-range correlations are found to significantly enhance the -y-spectra of the core subshells. For Ar, Kr and Xe, the core contributions to Zeff are found to be 0,55%, 1.5% and 2.2% respectively, their small values reflecting the difficulty for the positron to probe distances close to the nucleus. However, the core subshells have a broad momentum distribution and they markedly contribute to, and even dominate, the -y-spectra at Doppler energy shifts .2::. 3 ke V. It is found that proper inclusion of the core spectra is crucial in bringing the theoretical spectra into agreement with the experiment across the full range of Doppler energy shifts. State-dependent vertex enhancement factors 1nl, which quantify the effects of the short- range correlations, are estimated analytically and calculated using the MBT. They are found to follow a simple and physically motivated scaling with the subshell ionization energy Inz: 1nl = 1 + AI.;r;:z + sr], where A, Band (3 are positive constants. These factors can be incorporated in simple independent-particle-model calculations to reconstruct the true annihilation -y-spectra for annihilation on core electrons of atoms across the periodic table, and on the localized atomic-like core electrons of condensed matter systems. In the last part of the work, the atomic MBT is used to address the important problem of calculating the annihilation -y-spectra of molecues, with focus on the role of the positron- nuclear repulsion and correlation effects.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available