Positron Lifetime Spectra in Molecular Substances

Earlier quantum-mechanical model calculations show, and experiments have confirmed, that at low and intermediate temperatures the long lifetime of orthopositronium atoms ($o$-Ps) against electron pickoff in a molecular lattice rises with the free volume, because the overlap between the positron component of the $o$-Ps wave function and the wave function of the lattice electrons responsible for pickoff decreases with increasing lattice spacing. An extension of this model to higher temperatures, where the influence of lattice vibrations becomes important, predicts a shortening of the pickoff lifetime relative to its value in a cold lattice of the same free volume. In fact, under favorable conditions, on heating, the rise with free volume can be overshadowed by the decline due to thermal density fluctuations such that a maximum in the pickoff lifetime develops at a characteristic inversion temperature. In search for such maxima, lifetime spectra of positrons annihilating in three organic insulators (polyethylene, polytetrafluoroethylene, glycerol) were measured with high resolution over wide temperature ranges, and decomposed into three components. On analysis, the data reveal that the relative intensities of the components with the longest and shortest lifetimes, in their dependence on temperature and pressure, vary as the free volume. At elevated temperatures the longest lifetime in polytetrafluoroethylene exhibits a maximum which is interpreted to be a manifestation of the thermal motion of the lattice molecules.