Isotope-shift measurement of the NO2gap mode spectrum in KI with persistent ir spectral holes

Abstract
Persistent infrared spectral holes are used to identify the isotope shifts of the far-infrared gap modes associated with NO2 in KI. Two of the modes are determined to be translational while the third has a mixed translational-librational character. The properties of the mixed mode disagree in a fundamental way with the predictions of the previous lattice-dynamical calculation for this defect-lattice system.