Localised vibrations due to pairs and triplet clusters of the NO2-ions in KI single crystals

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Abstract
The authors report here high-resolution infrared measurements at 1.7K on KI single crystals doped with NO2- ions. At very low concentrations of approximately 1017 NO2- ions cm-3, they have observed a multiplet structure with the antisymmetric stretching vibration nu 3 of the NO2- ion arising due to tunnelling motion of the ion among the twelve equilibrium potential wells in the KI lattice. At relatively higher concentrations of the NO2- impurities (10-19 ions cm-3), several extra sharp and distinct bands have been observed in the region of the antisymmetric stretching vibration of the NO2- ions in KI at 1.7K. This structure has been interpreted in terms of mutual interaction effects between the transition dipole moments during the nu 3 vibrations of the nearby nitrite ions in pairs and triplet clusters. Several ion pairs and triplet clusters were considered and their vibrational spectrum has been calculated using a model of coupled harmonic oscillators. From these studies, the authors have obtained the magnitude of the transition dipole moment mu i=0.248 D during the nu 3 vibration of the NO2- ion doped in KI.