Chemical structural effects on γ-ray spectra of positron annihilation in fluorobenzenes*
eChemistry Laboratory, Faculty of Life and Social Sciences,
Swinburne University of Technology, P.O. Box 218, Hawthorn, 3122
2 School of Physics, Ludong University, Yantai, Shandong 264025, P.R. China
3 Centre for Theoretical Atomic, Molecular and Optical Physics, School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, Northern Ireland, UK
4 Physics Department, University of California, San Diego, La Jolla, 92093-0319 California, USA
a e-mail: email@example.com
Received in final form: 5 March 2012
Published online: 30 April 2012
Spectra of γ-ray Doppler shifts for positron annihilation in benzene and its fluoro-derivatives are simulated using low energy plane wave positron (LEPWP) approximation. The results are compared with available measurements. It is found that the Doppler shifts in these larger aromatic compounds are dominated by the contributions of the valence electrons and that the LEPWP model overestimates the measurements by approximately 30%, in agreement with previous findings in noble gases and small molecules. It is further revealed that the halogen atoms not only switch the sign of the charges on carbon atoms that they bond to, but that they also polarize other C-H bonds in the molecule leading to a redistribution of the molecular electrostatic potentials. As a result, it is likely that the halogen atoms contribute more significantly to the annihilation process. The present study also suggests that, while the Doppler shifts are sensitive to the number of valence electrons in the molecules, they are less sensitive to the chemical structures of isomers that have the same numbers and type of atoms and, hence, the same numbers of electrons. Further investigation of this effect is warranted.
Key words: Topical issue: Electron/Positron Collision. Guest editors: Michael Brunger, Anne Lafosse, Gaetana Laricchia, Paulo Limao-Vieira and Nigel Mason
Figure 1S and Table 1S are only available in electronic format at www.epj.org.
© EDP Sciences, Società Italiana di Fisica and Springer-Verlag 2012