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One of the remarkable effects observed with a change in temperature is the appearance of a maximum in the positronium (Ps) rate constant K&sd in the reaction with an acceptor (Ac). The analysis of these dependences in liquids of different polarity has been made. It has been found that the temperature T(K,,) corresponding to the maximum value of rises when the polarity increases. The interpretation of the experimental data based on certain assumptions has been proposed. These assumptions are that (1) in the positronium reactions with the acceptor the electron transfer from Ps to Ac takes place according to Ps + Ac -e,+ + Ac; and (2) the possibility of the electron transfer is due to thermal fluctuations in the solvent. At high temperatures the Kobsd value is determined by the probability of the electron tunneling from Ps to Ac. With the increasing temperature the probability value tends to zero because of the following dependence of the reorganization energy Ereorg on temperature 5": Ereox -F (n > 1). In contrast, at low temperatures Kobd is controlled by diffusion.The rise of T (K,,,) with the increase of the solvent polarity is explained by the decrease of energy required to separate the reaction products to infinity. IntroductionThe investigation of the temperature and medium effects on the course of the positronium (Ps) reactions has attracted little attention so far, and that is why these effects have not yet received a close study. One of the most remarkable effects induced by the temperature changes is the appearance of a maximum in the observed reaction rate constant (Kobsd) of the positronium reaction with an acceptor (Ac)To account for this fact the authors in ref 2 and 4 suggested that the positronium chemical reaction with an acceptor proceeds through an intermediate complex stage (Ps-Ac), and the positronium complex formation process in a solution may be described as follows

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