PRESSURE EFFECT ON POSITRONIUM REACTIONS IN ORGANIC SOLUTIONS

The reaction rate constants of positronium (Ps) atoms with a series of nitrobenzene derivatives and nitromethane in toluene solutions have been determined as a function of external pressure up to 1000 kg cm-2 (about 1000 atm). The rate constants for relatively weak Ps acceptors such as p-nitroanisole, 1-3-dimethyl-2-nitrobenzene and nitromethane increased considerably with external pressure, while those for relatively strong Ps acceptors like p-dinitrobenzene or nitrobenzene either decreased or changed little with external pressure. Comparison of the experimental results on pressure effects with those on temperature effects revealed that the application of external pressure suppresses the thermal decomposition of Ps complexes which are unstable at room temperature. The effect on a solvent of the Ps reaction was also studied by determining the reactivities of Ps atoms with nitrobenzene in various solvents as a function of external pressure.