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Title

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

Author(s)

Green, D. G.;  Saha, S.;  Wang, Feng;  Gribakin, G. F.;  Surko, C. M.

Abstract

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.

Publication type

Journal article

Research centre

Swinburne University of Technology. Faculty of Life and Social Sciences

Source

New Journal of Physics, Vol. 14, no. 3 (Mar 2012), article no. 035021

Publication year

2012

FOR Code(s)

02 Physical Sciences

Keyword(s)

Atomic and molecular physics;  Computational physics;  Density-functional theory;  Electron and positron scattering;  Positron scattering

Publisher

Institute of Physics

ISSN

1367-2630

Publisher URL

http://dx.doi.org/10.1088/1367-2630/14/3/035021

Copyright

Copyright © 2012 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft. The published version is reproduced in accordance with the terms of the Creative Commons Attribution-Non-Commercial-ShareAlike 3.0 licence (http://creativecommons.org/licenses/by-nc-sa/3.0/).

Full text

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