Assignments of the Two-Phonon Infrared Absorption Spectrum of LiF

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Abstract
Measurements of the far-infrared lattice absorption in LiF at several temperatures are reported. An approximate calculation of the relative two-phonon absorption intensity has been made considering the anharmonic mechanism only and using the lattice-dynamical data, resulting from a modified deformation-dipole model, of Karo and Hardy. The wave-vector density used was 64000 points per zone (1685 independent wave-vector positions). Such a density with a simple bin-sampling smoothing procedure gave a fairly good wave-number resolution in the computed two-phonon spectra. The agreement with the two-phonon difference absorption presented here and the summation results reported elsewhere is poor, but the discrepancies may be seen to be in the lattice-dynamical data. By identifying the phonon branches responsible for the features in the computed spectrum and considering various criteria necessary for strongly combining branches, it was possible to take the more accurate frequencies recently measured by inelastic neutron scattering and satisfactorily explain the experimental features. Phonon branches of less symmetry than the three major branches were found to contribute appreciably. No evidence was found for sizeable three-phonon absorption in the far infrared.

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