Angular correlation of positron annihilation radiation in

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 C$14, a weak electron acceptor. This difference causes probably a lower dispersion/polarization attraction energy of the molecules and Ps in C6Fl,. 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., C6F14), and then decreases to a low value, i.e., 0.1- 0.2, at higher electron affinity, 1