Vibrational relaxation of molecules in solids: The role of rotational and of translational local modes

A treatment of vibrational relaxation of impurity molecules in host solids is given, which introduces two improvements over existing theories: (i) The librational degree of freedom is taken into consideration. (ii) Local translational modes at the impurity site are included in the treatment. A realistic molecule-lattice interaction potential is employed. The result for the relaxation rate indicates that the contribution of local modes to the relaxation usually dominates over that of bulk modes. Another conclusion is that the rotational (librational) mode may be the most important receiving mode in the process if the rotational spacing is large. This strongly supports the indication provided by recent experiments on the importance of rotational modes in vibrational relaxation processes.