THE INTERACTION OF SLOW POSITRONS WITH ORGANIC-MOLECULES ABOVE AND BELOW POSITRONIUM FORMATION THRESHOLDS

A review is given of the ionization of organic molecules by monoenerge tic positrons having energies in the range of 0.5-15 eV. Two mechanism s, unique to positrons, are described If the kinetic energy of the pos itron is above the positronium formation threshold, such that electron s can be removed from the molecules to form free positronium atoms, th e ionization/fragmentation behavior can be explained qualitatively by a modification of the Ore gap theory. To explain how positrons can ion ize and fragment molecules when their kinetic energies are below the p ositronium formation threshold, it is necessary to assume that energy is transferred to the molecule by the annihilation process. Ionization cross sections far positrons having kinetic energies below the positr onium formation threshold are sensitive to molecular size, structure a nd bond types. Continuing work involves a search for positronium compo und formation and measurements of the kinetic energy distributions of ions.