Anomalous Dispersion Effects in Diffuse Reflectance Infrared Fourier Transform Spectroscopy: A Study of Optical Geometries

Abstract

In this report, the two most common diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) optical geometries (on-axis and off-axis) are investigated in terms of adherence to the Kubelka-Munk theory. It was found that specular reflection, whether in the form of regular Fresnel reflection or diffuse Fresnel reflection, is the major cause of spectral distortion in typical diffuse reflectance measurements. A discussion of the origin of the variation in specular background associated with resonances is presented. Once the adverse effects of specular reflection are minimized, the linear relationship between response and concentration predicted by Kubelka-Munk theory was found to extend to concentrated samples. Up to a point, this was the case even for intense absorption bands where anomalous dispersion leads to large changes in specular intensity.