Shape and structure of phonon sidebands of the ν1 stretching‐vibration absorption band of the NCO− impurity ion in alkali halide crystals

Abstract

AbstractExperimental data are compared of the shape and structure of the phonon sidebands (PS) of the ν1 intramolecular vibration (IMV) of the impurity ion NCO− in KCl, KBr, and KI matrices obtained at 10 °K with their theoretical calculations. It is shown that in f.c.c. host lattices the shape (distribution of absorption coefficient K(ωk)) of the acoustic branch of PS is in quantitative agreement with the expression magnified image × × 〈sin2 (κ, ι)〉 g (ωκ), where ωk is the frequency displacement from the centre of the IMV absorption band (frequency of lattice phonon), ωm the maximum frequency of the phonon spectrum of the matrix, g(ωk) the density of its phonon states, k the quasimomentum of matrix phonon, and l the vector of the nearest neighbour of the impurity. In the optical branch of PS the agreement is qualitative. Assuming a change of the force constants of the two nearest axial neighbours of the linear impurity conditions are obtained of the appearance of local vibrations (LV) separating into the gap of the phonon spectrum of the matrix crystal and above ωm· The treatment is given of a sharp structure observed at the short‐wave side of PS and in the region of PS corresponding to the frequency gap of the f.c.c. matrices (doublets). Corresponding limiting changes of the force constants in the impurity crystals are evaluated.