Spectroscopic properties and relaxation processes of impurity molecules in solids. I. Rotational spectra

The spectroscopic properties and relaxation processes of impurity diatomic molecules are studied theoretically. In order to treat the eigenvalue problem and the relaxation process consistently, we propose a unified theory based on the adiabatic approximation for the system of a diatomic molecule in monoatomic crystal. The adiabatic approximation is used to separate the total number of degrees of freedom of the system into the intramolecular vibrational, rotational, and lattice vibrational degrees of freedom. In this paper, we shall report the calculation of the rotational level shifts for HCl in rare gas matrices, using the M5 potential proposed by Hutson and Howard. The effect of lattice vibration is appropriately taken into consideration by means of the Green's function method for point defects developed by Maradudin et al. The agreement between our theory and experimental results is good. It will be shown that the coupling between the molecular rotation and lattice vibration is important, which improves the idea of the cell model in which the effect of host atoms is completely neglected.