Defects in molecular solids

A short review of experimental work on positrons in the defects of molecular solids is presented. A more or less clear picture is obtained for crystalline solids with impurities and for plastic crystals containing vacancies. There are also some data about the vacancies in the rotational phase. The nature of a long-lived component appearing in brittle crystals at high temperatures is discussed. The data about other defects are not numerous. A brief description of intramolecular defects and their possible role in Ps formation is given. In spite of all complexity and not always clear nature of the observed effects, the positronium formation and trapping on defects in organic crystals seems worth of further investigations. (author)