Abstract
Motional states of the NO2- ion doped in KI single crystals have been investigated using high-resolution infrared techniques. At very low concentrations of the impurity, the antisymmetric stretching vibration ( nu 3) of the NO2- ion shows multiplet structure at 1.7K. This structure can be understood in terms of contributions of different types of tunnelling motions of the NO2- ion among the twelve equivalent potential wells in the KI lattice. An attempt has been made to give an estimate of the potential barrier hindering reorientation of the ion in the KI lattice.