Angular Dependence of Diffuse Reflectance Infrared Spectra. Part II: Effect of Polarization

Abstract

The magnitude of specular and diffuse Fresnel reflectance from powdered samples was investigated with the use of an FT-IR spectrogoniophotometer with wire-grid polarizers mounted in front of and behind the sample. Specular Fresnel reflectance maintains the polarization of the incoming infrared beam and can therefore be eliminated by crossing the orientation of the second polarizer relative to the first. For neat samples, diffuse Fresnel reflectance was found to be only slightly affected by the presence of the two polarizers. While specular Fresnel reflectance is affected by the optical geometry used in making the reflectance measurement, diffuse Fresnel reflectance is unaffected by optical geometry and can only be eliminated by dilution of the neat sample. For dilute samples, the optical geometry and the orientation of the two polarizers slightly affect the absolute intensity of absorption bands, but do not change relative band intensities or band positions.