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We investigate the modeling of positronium (Ps) states and their pick-offannihilation trapped at open volumes pockets in condensed molecular matter. Ourstarting point is the interacting many-body system of Ps and a He atom becauseit is the smallest entity that can mimic the energy gap between the highestoccupied and lowest unoccupied molecular orbitals of molecules and yet the themany-body structure of the HePs system can be calculated accurately enough. Theexact-diagonalization solution of the HePs system enables us to construct apair-wise full-correlation single-particle potential for the Ps-He interactionand the total potential in solids is obtained as a superposition of thepair-wise potentials. We study in detail Ps states and their pick-offannihilation rates in voids inside solid He and analyse experimental resultsfor Ps-induced voids in liquid He obtaining the radii of the voids. Moreimportantly, we generalize our conclusions by testing the validity of theTao-Eldrup model, widely used to analyse ortho-Ps annihilation measurements forvoids in molecular matter, against our theoretical results for the solid He.Moreover, we discuss the influence of the partial charges of polar moleculesand the strength of the van der Waals interaction on the pick-off annihilationrate.

Pick-off annihilation of positronium in matter using full correlation single particle potentials: solid He