Infrared Absorption by Localized Vibrations of H− and D− Ions in NaF and LiF Crystals

Abstract

AbstractThe infrared absorption due to localized vibrations of H− and D− ions in NaF and LiF is measured between 20 and 300 °K. The oscillator strengths are temperature independent. Measurements with uniaxial pressure show that the frequency of the local mode in NaF:H− is shifted by lattice distortions of the symmetry of Γ and Γ but not of Γ. In NaF:D−, LiF:H−, and LiF:D− two‐phonon decays are allowed resulting in relatively large halfwidths at low temperatures. In NaF:H− a two‐phonon decay is not possible and the halfwidth becomes smaller than the minimum spectral slit width of the grating spectrometer. The temperature dependence can be described by a scattering process in NaF:H− and by a composition of a scattering process with a two‐phonon decay process in the other systems. The anharmonic frequency shift is calculated to a first approximation. In a simple density approximation the side band spectra can be described better by anharmonicity than by nonlinear dipole moments.