MECHANISM OF POSITRONIUM FORMATION IN MIXTURES OF FLUORINATED BENZENES AND n-HEXANE.

Positronium yield (I//3) in binary mixtures of benzene derivatives (p-difluorobenzene, 1,2,4,5-tetrafluorobenzene, pentafluorobenzene, perfluorobenzene and p-xylene) in n-hexane has been measured. As additives all of the aromatic compounds have been found to enhance Ps yield in n-hexane, but the effect was unexpectedly smaller for 1,2,4,5-C//6H//2F//4. It is proposed that there are at least three processes which lead to the enhancement: anti-recombination by scavenging excess electrons, anti-recombination by scavenging positive holes, and electron abstraction from excited molecular states. Addition of a small amount of n-hexane in the fluorinated benzenes resulted in a decrease in the Ps yield to a different degree which strongly depended on the number of fluorine atoms substituted. Discussion is made on the Ps formation mechanism taking into account possible role of excited molecular states.