Reststrahlen Effects in Drift Experiments of Alkali Metal Halides in the Far-Infrared Region

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 reststrahlen 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 v TO and v LO phonon regions, with the intensity relation being v TO > v LO . The inverse mode at the low-frequency region near the v TO mode was found always at higher energy than the v TO 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 v LO mode as determined by the Lyddane-Sachs-Teller equation. A Kramers-Kronig dispersion analysis was conducted and the changes in the index of refraction ( n) and the absorption coefficient ( K) were determined for various points around the dispersion frequency.