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Conclusion  

This has been the most future-oriented workshop in my memory. Many participants are thinking about the next experiments to perform, new phenomena to study, and new parameters to measure. We look forward to new optical correlations, both with exciting radiation and intrinsic fluorescence. We very badly need positron and positronium binding energies to atoms, molecules, and radicals. We very badly need work functions, band gaps and edges, and trap depths for positron and positronium in molecular liquids and solids. The binding energies of positronium itself in various molecular materials is an important and poorly understood quantity. New data presented at this workshop on the energy of positronium at its formation, and its thermalization time, are most significant,3 and we look forward to refinements in those measurements and in their interpretation. We need to understand the thermalization times of positrons much better than we do. The role of voids and/or surfaces in positronium formation needs to be illuminated, including numerical estimates and measurements for the cross section of this most important process.Finally, a note of thanks and appreciation to the hard-working organizers of this workshop, without whose selfless labors we would not have enjoyed the many benefits of talking face to face with our colleagues from far away.

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Abstract  

A review is given of the ionization of organic moecules by monoenergetic positrons having energies in the range of 0.5–15 eV. Two mechanisms, unique to positrons, are described. If the kinetic energy of the positron is above the positronium formation threshold, such that electrons can be removed from the molecules to form free positronium atoms, the ionization/fragmentation behavior can be explained qualitatively by a modification of the Ore gap theory. To explain how positrons can ionize and fragment molecules when their kinetic energies are below the positronium formation threshold, it is necessary to assume that energy is transferred to the molecule by the annihilation process. Ionization cross sections for positrons having kinetic energies below the positronium formation threshold are sensitive to molecular size, structure and bond types. Continuing work involves a search for positronium compound formation and measurements of the kinetic energy distributions of ions.

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