Positron lifetime spectra were determined, between room temperature and 40K, for various organic compounds of low molecular weight, and for normal and cyclic hydrocarbons and their perfluorinated versions. Information concerning vacancies was deduced from the lifetime and intensity of the ortho-positronium component. The vacancy sizes were found to be greater in solids that were composed of larger molecules. This suggested that ortho-positronium could reveal the vacancy size. In the case of perfluorocyclohexane, the vacancy size was greater than expected. In all of the solids, 2 ortho-positronium states were observed whose relative importance could easily change and lead to a hysteresis-like behavior. At temperatures of about 40K, the ortho-positronium state with a longer lifetime and greater intensity became overwhelmingly predominant in all of the materials. It was found that the ortho-positronium lifetime and intensity were not very sensitive to most phase transitions; including the melting point.

Y.Ito, H.F.M.Mohamed, M.Shiotani: Journal of Physical Chemistry, 1996, 100[33], 14161-5