Reaction rate approach to dipolar relaxation in alkali halides: Adiabaticity versus classical, activated-tunneling, and quantal dipoles

Abstract

This paper is aimed at presenting a simple vibronic model for describing the dipolar reorientation in crystals by means of reaction rate theory. The Hamiltonian of an isolated dipole is simplified so as to render the problem solvable. Depending on the crossover splitting the dipoles may reorientate adiabatically with a high electron-transfer expectancy or exhibit low reorientation rates due to low expectancy. An important quantity to distinguish between adiabatic dipoles behaving classically and ones reorientating by means of quantum-mechanical tunneling is Christov's characteristic temperature which is found to relate to the barrier height and crossover splitting. ITC data on impurity-vacancy dipoles in Eu-doped alkali halides are reanalyzed.