Rate processes in condensed media: The role of anharmonicity

We discuss the properties of a model which describes a variety of phenomena such as excitation migration in solids (exciton), electronic conductivity of organic crystals (polaron), electron transfer reactions, isomerization reactions, radiationless transitions, and motion of chemisorbed atoms. It is shown that, by taking into account the anharmonicity of the medium, the temperature dependence of the rate of the processes enumerated above can change substantially. In the case of the polaron and exciton, the Arrhenius temperature dependence is changed by the anharmonicity into a weak temperature dependence, which is in agreement with the experimental observations. For radiationless transitions and electron transfer reactions, the equations obtained by considering harmonic vibrations are in agreement with the experimental data and we conclude that the anharmonic effects in these cases are small. In the case of the isomerization reactions or migration of chemisorbed atoms, the anharmonicity does not change the form of the equation enough to allow one to distinguish experimentally between the two cases. The interpretation of the activation energy and the temperature dependence of the pre-exponential factor are, however, changed by strong anharmonicity.