Infrared reststrahlen revisited: commonly disregarded optical details related to n<1.

Spectral ellipsometry has developed into a routine method applicable to the infrared spectral range. It can give both the refractive index (n) and the absorption index (k), which has so far only been determined for a limited number of compounds. It turns out that vibrations that have a dispersion with an interval of n<1 are by no means restricted to crystals, but occur with numerous compounds including polymers and liquids. In conventional infrared spectroscopy, one usually is not aware of such a situation and so the consequences are disregarded. These include the so-called reststrahlen (residual rays) bands and specific phenomena that occur when n matches the absorption index (the Berreman effect) or the refractive index of ambient air (vanishing reflection, the Christiansen effect). These and some typical applications are discussed, including model calculations and experimental data.