Many-body theory of positron-molecule interactions

Abstract

Diagrammatic many-body theory (MBT) is used to study positron interactions with molecules. Calculations are performed for positron binding to molecules, Doppler-broadened γ-spectra for positron annihilation on molecules, and positronic bonding in systems of dianions. Implemented in the state-of-the-art massively parallelized C++ code EXCITON+ [J. Hofierka et al. Nature 606 688 (2022)], the approach uses the Feynman diagram technique to construct a positron-molecule self-energy that accurately accounts for important electron-positron correlations including polarization of the target by the positron, screening of the Coulomb interaction, electron-hole interactions, virtual positronium formation and positron-hole interactions.

Chapters 3 and 4 present positron binding energy calculations for halogenated hydrocarbons and heterocyclic ring molecules respectively. Very good agreement with experiment is found (to within 1 meV in cases), especially for the ringed molecules where blind comparisons with experiment are made. For halogenated hydrocarbons, the general effect of halogenation on the binding energies is quantified and explained. For the ringed molecules, the interplay between permanent dipole moments and molecular π-bonds in determining the localization and shape of the correlated positron Dyson wavefunction is highlighted.

Chapter 5 extends the present MBT to the calculation of Doppler-broadened γ-spectra. The equations for γ-spectra in Gaussian bases are presented, and calculations are performed using positron Dyson wavefunctions and the annihilation vertex in the independent particle approximation, augmented by enhancement factors. The calculated spectra for molecules are found to be too broad relative to experiment, but the present MBT-based 0th-order implementation provides a framework for including the full annihilation vertex in molecular calculations in the future.

In Chapter 6, the MBT is used to study the formation of positronic bonds in systems of otherwise repelling anions. The calculated positronic bond lengths and bond energies for H_2^{2−}, F_2^{2−} and Cl_2^{2−} from previous studies are closely reproduced. Entirely new predictions of positronic bonding are made for the (CN)_2^{2−} and (NCO)_2^{2−} systems.

Date of Award	Dec 2024
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Northern Ireland Department for the Economy
Supervisor	Dermot Green (Supervisor) & Myrta Grüning (Supervisor)