On positron studies of molecular crystals

A brief review is given of some recent results of positron annihilation studies of molecular crystals, mainly concentrated around the influence of defects on the measured annihilation characteristics. Ice, which has been investigated in considerable detail, seems to provide a relatively simple picture of positronium behaviour in molecular crystals and is therefore discussed first. The deviations from this simple picture by other molecular crystals are discussed, and problems in understanding certain aspects of the experimental observations are pointed out. Effects of temperature and impurity doping are illustrated. Finally the special class of plastic crystals is treated, illustrating e.g. the determination of vacancy formation energies and of relationships between o-Ps lifetimes and molecular packing as well as vacancy volumes. A conclusion may be that in spite of the short-comings in the detailed understanding of the Ps states in molecular crystals, the positron annihilation techniques are able to give new information about defects, especially of vacancy type, in such crystals. (Auth.)