INFRARED ABSORPTION BY LOCALIZED VIBRATIONS OF H- AND D- IONS IN NAF AND LIF CRYSTALS

The 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 1+ and Γ 3+ but not of Γ 5+. 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. Copyright © 1969 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim