Effect of atomic electric quadrupole moment on positron binding

TL;DR

This paper investigates how the electric quadrupole moment influences positron binding to atoms and molecules, showing that large quadrupole moments can enable binding even without polarization effects, with implications for excited states and molecules.

Contribution

It demonstrates that sufficiently large electric quadrupole moments can enable positron binding independently of polarization, highlighting their significance in excited atoms and molecules.

Findings

Quadrupole moments >50 a.u. can bind positrons without polarization.

In certain excited states, quadrupole effects determine stability.

Most atoms have negligible quadrupole influence on positron binding.

Abstract

Effect of the electric quadrupole moment, $Q$, is studied for positron-atom bound systems. It is demonstrated that for $Q >50$ a.u. the electric quadrupole potential is sufficiently strong to bind positron (or electron) even in the absence of the dipole polarization potential. Such large values of $Q$ are not known for atomic ground states, however, they exist in molecules and excited atoms. In the state $2s2p~^3P^o_2$ of beryllium, the quadrupole contribution makes difference between stable bound state and decay to Be$^+$ ion and positronium. In a majority of atoms the quadrupole contribution is small and can be neglected.