Positrons in doped pyrene

Abstract

It was shown that the annihilation of free positrons produces a momentum distribution of annihilating pairs which is narrower for doped pyrene than for pure pyrene. We propose to explain this effect by enhanced annihilation on guest molecules; the enhancement factor for phenanthrene as admixture is of the order of 25. The increased annihilation probability on admixtures is not correlated with the respective effective numbers of electrons participating in annihilation per molecule (Z_eff) observed in collisions of slow positrons with free molecules in vacuum.