Multiphonon absorption in ionic crystals

Publisher Website
Google Scholar
Abstract
Calculations of the frequency and temperature dependence of infrared multiphonon absorption for the alkali halides and alkaline-earth flourides are compared with previously published data and some new results. The temperature dependence of the infrared absorption of pure crystals of NaF, NaCl, and KCl at 10.6 μm from room temperature to close to the melting point are reported. The calculations are based upon a simple approach involving and interatomic Morse-potential function in which lattice dispersion is introduced via a multiphonon frequency distribution. Good agreement between experiment and theory is obtained in which the "observed" lattice density-of-states and thermal-expansion-coefficient data (which determine the degree of anharmonicity) are the principal input to the calculations. Some structure in the multiphonon spectrum is predicted for some compounds at low temperatures, which has not yet been observed expermentally. Since the Morse potential has an exact quantum-mechanical solution, the anharmonicity is contained in the calculations without resorting to perturbation theory.