The role of excited states on positronium formation and annihilation in nonpolar organic systems
Molecular excited states are formed in solution from the interaction of ionizing radiation with the medium, both from direct excitation (coulombic interaction), as well as from geminate ion pair recombination. Electron scavenging, positive hold scavenging, energy transfer to an acceptor, and quenching reactions by solutes can alter geminate recombination and excited state processes, thereby altering the observed effects of the radiolysis. The presence of solutes also affects the lifetime and formation of positronium. The spur model of positronium formation takes into account the effects of electron scavenging on removing the channel for positronium formation, but only recently has positronium formation from molecular excited states been considered. The modified spur model, which includes the hot-atom model, rationalizes differences between radiation and positronium chemistry, whereby the predominating model is the one that best fits the observed results. This dual model, however, is not predictive. We have developed a kinetic scheme based on experimental studies using electron scavengers nitrobenzene, nitrocyclohexane, and hexafluorobenzene in aliphatic solvents in which radiation chemical data can be incorporated directly into positronium formation mechanisms. An additional pathway for positronium formation in systems containing efficient electron scavengers has been found to be necessary to produce the amount of positronium observed experimentally. The mechanism proposed is positronium formation by anion pick-off, and rate constants for this reactions have been calculated. No direct interaction of molecular triplet excited states generated photolytically with ortho-positronium has been observed, contrary to previously reported results. Anomalous quenching of ortho-positronium by an external magnetic field has been observed in n-hexane solutions containing nitrobenzene. A triplet radical cage formed from energy transfer to nitrobenzene by an excited state produced in geminate recombination has been proposed to account for this phenomenon. This mechanism has been incorporated into the kinetic scheme and the dynamics of this interaction have been modelled. Temperature effects on this system, as well as temperature and magnetic field effects on other electron scavengers and solvents, have been investigated. The kinetic schemes developed in this work can be used to gain insight into early spur processes, and are complementary to the radiation chemistry pulse radiolysis results.
David A Diehl,
"The role of excited states on positronium formation and annihilation in nonpolar organic systems"
(January 1, 1988).
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