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Positronium atoms (Ps) are widely used as a probe to characterize voids orvacancies in non-metallic materials, where Ps annihilation lifetime is stronglymodified by pickoff, depending on the size of the trapping cavity and on theappropriate outer electron density. The connection between these materialcharacteristics and Ps annihilation lifetimes is usually based on models thatdo not consider the requirements of full electron indistinguishability, whichmust be taken into account for a correct description of pickoff annihilationprocesses. In this report we provide a formal theoretical framework in whichexchange effects between Ps and surrounding electrons are introduced in anatural way, giving a clear and versatile picture of the various contributionsto the Ps pickoff annihilation. Moreover, our results provide a simpleexplanation of the lowering of the contact density (the Ps-electron density atthe positron position) as a direct consequence of the electronsindistinguishability, at variance with previous interpretation based on spatialdeformations of Ps wavefunction. Calculations are performed within the"symmetry adapted perturbation theory" approach, and the results are comparedwith experimental data on Ps lifetimes of some polymers and molecular solids.Finally, introducing suitable approximations, we also recover early modelingand give a simple interpretation of Ps properties in subnanometric voids.

Formal calculation of exchange effects on confined positronium