Paper Title:

Calculation of Gamma Spectra for Positron Annihilation on Molecules

Periodical Materials Science Forum (Volume 666)
Main Theme Progress in Positron Annihilation
Edited by Radosław Zaleski
Pages 21-24
DOI 10.4028/www.scientific.net/MSF.666.21
Citation D.G. Green et al., 2010, Materials Science Forum, 666, 21
Online since December 2010
Authors D.G. Green, S. Saha, F. Wang, G.F. Gribakin, C.M. Surko
Keywords Electron Momentum Density, Gamma Spectra, Molecules, Positron Annihilation
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Calculations of gamma spectra for positron annihilation for a selection of molecules, including methane and its fluoro-substitutes, ethane, propane, butane and benzene are presented. The contribution to the -spectra from individual molecular orbitals is obtained from electron momentum distributions calculated using the density functional theory (DFT) based B3LYP/TZVP model. For positrons thermalised to room temperature, the calculation, in its simplest form, effectively treats the positron as a plane wave and gives positron annihilation  spectra linewidths that are broader (30–40%) than experiment, although the main chemical trends are reproduced. The main physical reason for this is the neglect of positron repulsion from the nuclei. We show that this effect can be incorporated through momentum-dependent correction factors, determined from positron-atom calculations, e.g., many-body perturbation theory. Inclusion of these factors in the calculation gives linewidths that are in improved agreement with experiment.