Infrared Lattice Absorption by Gap Modes and Resonance Modes in KI

Impurity-induced infrared absorption in KI containing Cl^- ions as substitutional impurities has been observed. A sharp line appears in the absorption spectrum in the neighborhood of 77 cm^-1, and at least three broad lines are observed near 61 cm^-1. The results of a theoretical calculation show that the identification of the former absorption peak with a localized mode whose frequency lies in the gap in the frequency spectrum of KI between the acoustic and optical branches is compatible with the experimental results. Similarly, the broad absorption peaks near 61 cm^-1 can be identified with resonance modes in the acoustic continuum.