The infrared optical constants of NaCl are calculated by applying an extended form of the Born and Blackman anharmonic potential theory to the Kellerman model of the NaCl crystal. A damping factor is derived which depends on the amplitudes and frequencies of the characteristic modes as well as on the frequency of the incident radiation. By a numerical sampling of wave vector space, this damping factor and the resultant optical constants are calculated in the range 25mgr;mdash;500mgr;. The theory predicts a subsidiary absorption maximum near 100mgr; which agrees with recent experimental measurements. The frequency distribution function of NaCl is also given here.
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