Authors: J.P. Sullivan, L.J. Barnes, J.P. Marler, S.J. Gilbert, C.M. Surko

435

Authors: J.A. Young, C.M. Surko

Abstract: At incident positron energies below the threshold for positronium atom formation, there are many cases in which annihilation rates for molecules are far in excess of that possible on the basis of simple two-body collisions. We now understand that this phenomenon is due to positron attachment to molecules mediated by vibrational Feshbach resonances. The attachment enhances greatly the overlap of the positron with molecular electrons and hence increases the probability of annihilation. Furthermore, measurements of the annihilation spectra as a function of incident positron energy provide a means of measuring positron-molecule binding energies. In this paper we present an overview of our current understanding of this process, highlighting key results and discussing outstanding issues that remain to be explained.

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Authors: Xiang Lei Chen, Wei Feng Guo, Huai Jiang Du, Hui Min Weng, Bang Jiao Ye

Abstract: Superposed-neutral-atom model and the finite-difference method (SNA-FD) are used to calculate the positron lifetime of different sizes and types of vacancy clusters in Ni3Al alloy. The calculated positron bulk lifetime in Ni3Al agrees well with the experimental results mentioned in literatures. The positron lifetimes of vacancy clusters with the same amount of vacancy but different components are also calculated. The results show that the positron lifetime is different when its components are different. The positron lifetimes of vacancy clusters with a single element but different sizes are also calculated. The result shows that the positron lifetime is almost a constant when its component is only Al vacancy, but the positron lifetime will increase at first and trend to be a constant when its component is only Ni vacancy.

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Authors: D.G. Green, S. Saha, F. Wang, G.F. Gribakin, C.M. Surko

Abstract: 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.

21

Authors: Kengo Shibuya, Haruo Saito

Abstract: We have investigated positronium‒xenon collisions at energies below 100 meV to find a strong temperature dependences of the annihilation rates. A partial wave analysis based on a modified effective range theory (MERT) is tested to explain the temperature dependences and to find significant contributions of the* p*-wave scattering component. The fact that MERT works well for analyzing positronium‒xenon collisions indicates that positronium is polarized during the collisions as proposed by other theoretical and experimental researchers.

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