Abstract
The electron density at the positron (contact density) in the ground state positronium (Ps) formed in condensed matter is generally found to be lower than in vacuum. This is usually attributed to microscopic electric fields which polarize Ps, by acting on the two particles of the atom. In this paper we quantitatively investigate an opposite effect. It is due to the confinement of Ps in small cavities existing in the host solid (e.g. free volume in polymers), which increases the contact density. Although this phenomenon is greater, the smaller is the size of the cavity, Ps polarization seems to play anyway a predominant role.
Highlights
The bound electron-positron system, positronium (Ps), formed in condensed matter shares with Ps in vacuo various features; the most important ones are the presence in the ground state of two sublevels characterized by different lifetimes and an energy separation between them [1]
There are significant differences, first of all the possibility to annihilate with an external electron in a relative singlet state (‘pickoff’ process), which changes the lifetimes with respect to those in vacuum (142 ns and 0.125 ns for o-Ps and for pPs, respectively); in particular, the o-Ps lifetime in condensed matter can be shortened up to a few ns [2]
A value of the relative contact density less than unity has been attributed to polarization effects on Ps [4,5] due to local electric fields acting in opposite way on the pair; this effect is expected to be relevant in the presence of polar media [6]
Summary
The bound electron-positron system, positronium (Ps), formed in condensed matter shares with Ps in vacuo various features; the most important ones are the presence in the ground state of two sublevels (triplet: ortho-Ps, o-Ps, parallel spins of the electron and positron; singlet: para-Ps, p-Ps, antiparallel spins) characterized by different lifetimes and an energy separation between them [1]. There are significant differences, first of all the possibility to annihilate with an external electron in a relative singlet state (‘pickoff’ process), which changes the lifetimes with respect to those in vacuum (142 ns and 0.125 ns for o-Ps and for pPs, respectively); in particular, the o-Ps lifetime in condensed matter can be shortened up to a few ns [2] Another feature generally found for Ps in matter is a different value, with respect to vacuum Ps, of the ‘contact density’, that is, the electron density at the positron, represented by Dy(0)D2, where y is the Ps wavefunction. In the present work we used the word ‘cavity’ to mean a generic volume of the solid structure in which Ps can be formed or trapped
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