Theory of rotational polaron states of a diatomic impurity molecule in an atomic crystal

Rotational polaron states of a diatomic substitutional impurity in an atomic lattice are analyzed in the framework of a consistent quantummechanical approach in the limits of strong, weak, and intermediate coupling. Calculations are made of the contributions to the free energy of the system due to the renormalization of the rotational constant of a braked rotator and tunnel splitting of the librational energy levels. It is shown that there is a temperature region in which there are important corrections to the thermodynamic functions associated with radiative friction exerted on the braked rotator by the phononsubsystem.