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The optical modes of vibration in an ionic crystal of finite thickness are found for wavelengths long compared to the interionic spacing. Retardation of the Coulomb interaction between the ions is included by solving the complete set of Maxwell's equations for the electromagnetic fields. Only the nonradiative modes, which have exponentially decreasing fields outside the slab, are discussed. The theoretical dispersion curves for various classes of modes in selected thicknesses of LiF are shown and are compared with the dispersion curves occurring without retardation.
Kliewer et al. (1966) studied this question.