DRIFT experiments in the far-infrared region are new. In the course of developing this technique, we have obtained some unusual results with the alkali metal halides due to the <i>reststrahlen</i> phenomenon. Neat solids (powder, single crystal, pressed pellets), as well as matrix (powdered polyethylene) diluted salts, gave inverse peaks (maxima in intensity on a percent T plot and a minima on an absorbance plot). The use of the blocker device on the diffuse reflectance attachment failed to normalize the peaks. The inverted peaks centered in the ν<sub>TO</sub> and ν<sub>LO</sub> phonon regions, with the intensity relation being ν<sub>TO</sub> > ν<sub>LO</sub> The inverse mode at the low-frequency region near the ν<sub>TO</sub> mode was found always at higher energy than the ν<sub>TO</sub> mode obtained by thin film experiments. It also was accompanied by shoulders on the high- and low-frequency sides. The weaker, higher frequency band was found to occur close to the ν<sub>LO</sub> mode as determined by the Lyddane-Sachs-Teller equation. A Kramers-Kronig dispersion analysis was conducted and the changes in the index of refraction (<i>n</i>) and the absorption coefficient (<i>K</i>) were determined for various points around the dispersion frequency.

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