Angular correlation of positron annihilation radiation in perfluorohexane mixtures with nonpolar liquids

Abstract
Positron-annihilation linear-slit angular-correlation (AC) spectra in perfluorohexane (C₆F₁₄) mixtures with hexane and isooctane in the entire concentration interval are reported. Annihilation with electrons in fluorine results in an unusually broad AC-component. The data show that the free positron reacts to form a bound state with C₆F₁₄ at low C₆F₁₄ concentration. At high C₆F₁₄ concentration the positron in Ps annihilates preferentially with electrons in the hydrocarbons, and not with the electrons in C₆F₁₄ before this cannot be avoided. Hence, the energy of the Ps bubble state is lower if the surface consists of hydrocarbon molecules. While the positron in Ps penetrates further into the hydrocarbons than the electron in Ps, the two particles have probably about equal penetration in C₆F₁₄, a weak electron acceptor. This difference causes probably a lower dispersion/polarization attraction energy of the molecules and Ps in C₆F₁₄. It is conjectured that the Ps relative contact density increases from about 0.8 in hydrocarbons to roughly 1.0 - 1.1 in weak electron acceptor liquids (e.g., C₆F_l4), and then decreases to a low value, i.e., 0.1 - 0.2, at higher electron affinity.