Positron binding in molecules

The positron, which is the anti-particle of the electron, is now widely used in both scientific and technological areas. The detail mechanism of such processes, however, is still unclear in the molecular level. A positron affinity (PA) value, which is a binding energy of a positron to an atom or molecule, has now been experimentally measured by Surko and co-workers for many molecular species such as acetaldehyde, acetone, and acetonitrile organic molecules [1], based on the vibrational Feshbach resonance by incident low-energy positrons. Thus, in order to elucidate the mechanism of the positron binding to molecules, the theoretical analysis including the effect of molecular vibrations is indispensable.

In this study, we will show the effect of molecular vibrations on PA values, based on ab initio multi-component quantum Monte Carlo (QMC) [2] and molecular orbital (MC_MO) [3] methods for the electronic and positronic wave functions simultaneously. We have applied these methods to some organic molecules. We confirmed that PA variations arise from the change in both permanent dipole moment and dipole-polarizability. Also, we will show the positron interaction with unstable homonuclear anion dimer (X⁻)₂ to form the positronic bound state of [X⁻; e⁺; X⁻] (X = H and Li). For [H⁻; e⁺; H⁻] system, we confirmed that the bound state is formed by the positron intermediate structure, called the “positronic covalent bonding” [4]. Meanwhile, for [Li⁻; e⁺; Li⁻], we obtained that the bound state should have a different positronic bound structure at the short internuclear distance. We resolved these different stabilities with “positronium (complex between e⁺ and e⁻) binding abilities” of systems.