Abstract
The formation of positronium in benzene solutions containing a variety of halogenated compounds, such as CCl/sub 4/, CH/sub 2/Cl/sub 2/, C/sub 2/HCl/sub 3/, 1,1,1-C/sub 2/H/sub 3/Cl/sub 3/, o-C/sub 6/H/sub 4/CH/sub 3/Br, o-C/sub 6/H/sub 4/Cl/sub 2/, o-C/sub 6/H/sub 4/CH/sub 3/Cl, C/sub 6/H/sub 5/Cl, and other mono- and dihalobenzene, was studied in the presence and absence of C/sub 6/F/sub 6/ additives. The observed I/sub 2/ values, which are indicative of the number of thermalized positronium atoms formed, showed in each system a good correlation with dissociative electron attachment parameters of the solute species. C/sub 6/F/sub 6/ additives increased the number of Ps atoms reaching thermal energies. The experimental results are discussed in terms of a reaction scheme in which Ps is formed as a result of electron abstraction from the surrounding molecules by energetic positrons. These Ps atoms can either react rapidly with solute species if their excess kinetic energy favors such a reaction, or with species generated in the reactions of free electrons produced in the positron spur with solute molecules via dissociative electron attachment. The number of these reactive species is mainly determined by the number of free electrons having sufficient energies to undergo dissociative electron attachment. A compound suchmore » as C/sub 6/F/sub 6/, which has a high cross section for nondissociative electron attachment, competes for free electrons with the solute, and protects the latter from radiolysis. Thus in the presence of C/sub 6/F/sub 6/ fewer reactive species are formed in the spur with which the Ps can rapidly react and more Ps atoms will reachthermal energies. An alternative to this reaction scheme is the previously reported spur reaction model which assumes positronium formation via free positron and free electron combination in the positron spur. The results will be also discussed within the framework of this latter model. 12 figures, 1 table.« less
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