Role of rotational and translational local modes in vibrational relaxation in solids: A study of NH and ND in solid Ar

A quantitative model for vibrational relaxation of impurity molecules in host crystals is presented, and applied to the vibrational relaxation of NH and ND in solid Ar. Realistic impurity-host interaction and impurity-cage geometry are employed. It is shown that impurity rotation is the dominant receiving mode in the relaxation, and only a small fraction of the energy released goes directly into phonon modes. The results explain the faster relaxation of NH compared with ND and the temperature independence of the measured decay rates.