Temperature, pressure and solvent dependence of positronium reactivity toward nitrobenzene

Positronium (Ps) reaction rates, with weak acceptors(Ac) (such as nitrobenz ene) leading to the formation of Ps-Ac complexes, have long been known to e xhibit intriguing features: nonmonotonic temperature dependence (at varianc e with the usual Arrhenius behavior), considerable variation in characteris tics with different solvents, and anomalies with respect to external pressu re at ambient temperature (large changes in some media and hardly any in ot hers). We present precise measurements performed in our laboratory on the t emperature dependence of the reaction rate (kappa) for Ps toward nitrobenze ne in benzene as the solvent, determining in particular, the "turnover poin t" (or rather the region) carefully, a result which when taken in juxtaposi tion with earlier experimental findings by Kobayashi, vis-a-vis external pr essure dependence of the rate in benzene versus other solvents, enables us to provide a natural semiquantitative explanation in the context of the bub ble model of Ps in liquids.