Matter-positronium interaction: An exact diagonalization study of the He atom - positronium system

TL;DR

This study uses exact diagonalization to analyze a helium-positronium system, revealing how Ps interacts with He and affecting annihilation rates, which aids in modeling Ps behavior in matter.

Contribution

It provides an exact quantum mechanical analysis of the He-positronium system, clarifying the interaction and polarization effects at various distances.

Findings

Ps polarization increases as it approaches He nucleus.

Depletion of He electron density observed near Ps.

Overlap-based annihilation rate requires correction for electron depletion.

Abstract

The many-body system comprising a He nucleus, three electrons, and a positron has been studied using the exact diagonalization technique. The purpose has been to clarify to which extent the system can be considered as a distinguishable positronium (Ps) atom interacting with a He atom and, thereby, to pave the way to a practical atomistic modeling of Ps states and annihilation in matter. The maximum value of the distance between the positron and the nucleus is constrained and the Ps atom at different distances from the nucleus is identified from the electron and positron densities, as well as from the electron-positron distance and center-of-mass distributions. The polarization of the Ps atom increases as its distance from the nucleus decreases. A depletion of the He electron density, particularly large at low density values, has been observed. The ortho-Ps pick-off annihilation rate calculated as the overlap of the positron and the free He electron densities has to be corrected for the observed depletion, specially at large pores/voids.