Librational relaxation of molecular impurities in host crystals

A study is made of phonon-assisted transitions between closely spaced librational levels of an impurity molecule in a host crystal. The contributions to librational relaxation of direct, of Orbach-type and of Raman processes are evaluated as a function of temperature. It is found for a model case of NH(3π) in solid Ar, that below T = 16 K, the direct process dominates, while for T > 20 K relaxation takes place mainly by the Raman mechanisms. The calculation predicts a nanosecond time scale for the relaxation at 4 K, and a picosecond time-scale for the relaxation at T = 24 K.