Effect of Intermolecular Forces on the Annihilation of Positrons in a Nematic Liquid Crystal

Lifetime spectra have been measured for positrons annihilating in the liquid-crystal $p$-azoxyanisole. Four lifetime components appear when positrons annihilate in either the nematic mesophase or in the isotropic liquid phase, but in solid $p$-azoxyanisole the longest-lived component is absent. No abrupt changes in either lifetimes or intensities are observed across the nematic-isotropic transition; however, the solid-nematic transition produces changes in all annihilation parameters which are unattributable to the accompanying 0.1% density change. Annihilation mechanisms are proposed to account for the observed positron lifetime components and their dependence on phase. The degree of molecular orientation appears to have little direct influence on annihilation behavior, but the nature and strength of the intermolecular forces in different phases can greatly affect the positron lifetimes and intensities.