Many-body theory of positron binding to polyatomic molecules

Positrons are unique probes of matter, with applications in materials science (ultra-sensitive diagnostic studies of surfaces, defects and porosity), medical imaging (PET), astrophysics, molecular spectroscopy, and fundamental physics.

Positron interactions with matter are characterised by strong many-body correlations. They significantly modify scattering, and enhance annihilation rates by orders of magnitudes. They also make the theoretical description of positron-matter interactions formidably challenging.

We have developed a diagrammatic many-body description of positron-molecule interactions that takes ab initio account of the correlations, implemented in the state-of-the-art code EXCITON. We solve the Dyson equation for the positron quasiparticle wavefunction in a Gaussian basis, constructing the positron-molecule self-energy including the GW diagram (at RPA/TDHF/BSE levels of theory), describing polarisation, screening and electron-hole attraction interactions, the ladder series of positron-electron interactions that describes virtual positronium formation, and the ladder series of positron-hole interactions. We have used it to calculate binding energies for a range of polar and non-polar molecules, focusing chiefly on the molecules for which both theory and experiment exist, but also making predictions (e.g. of positron binding to DNA nucleobases, and of the effect of fluorination vs chlorination in hydrocarbons). Delineating the effects of the correlations, we show, in particular, that virtual-positronium formation significantly enhances binding in polar molecules, and moreover, that it can be essential to support binding in non-polar molecules.

Overall, we find the best agreement with experiment to date (to within a few percent in cases). We have recently developed and extended the method to the calculation of the positron scattering and annihilation gamma spectra in molecules providing insight that should support the development of fundamental experiments and the myriad of antimatter-based technologies and applications.

Moreover, the positron-molecule problem provides a testbed for the development of methods to tackle the quantum many-body problem, for which our results can serve as benchmarks.