Spectroscopy in Infrared by Reflection and Its Use for Highly Absorbing Substances*

Abstract

The infrared spectra of many strongly-absorbing solids and liquids can be measured directly only with difficulties, involving preparation of very thin sections, or mulling the substance with diluents, etc. In such cases, the measurement of reflection offers an advantage. If the measurements are performed at two different angles of incidence, the two optical constants, namely, the refractive index n and the index of absorption κ of the studied substance can be obtained. A graphical method for obtaining n and κ from the measured reflecting powers is described and due corrections for the apparatus polarization are given. An alternative method for obtaining the optical constants from a single set of measurements at normal incidence is indicated for a case of a simple resonance band. This method is based on the impedance concept of the reflecting medium. Experimental arrangement for reflection measurements, using a standard infrared spectrometer, is described. Examples of measured reflecting powers and optical constants derived from them are given for the cases of quartz, mica, and liquid carbon tetrachloride.