Diffuse reflectance infrared spectroscopy: an experimental measure and interpretation of the sample volume size involved in the light scattering process

Abstract

This work presents the second part of a basic study of diffuse reflectance spectroscopy through a comparison of experimental observations with existing theories of diffuse reflectance of non-absorbing powders. The diffusion volume approximate shape has been assumed to be a cylinder. Its dimensions have been measured for different non-absorbing powders with a refractive index from 1.4 to 3.4, and with different distributions of particle size larger than the wavelength. The lateral extension diameter Φ m of this volume is at least two times larger than that of the irradiated sample surface. The variation of the cylinder height corresponding to the penetration depth, z ∞, has a behavior opposite to that of the powder diffusion coefficient, S. Its variation with the mean particle diameter is smaller than predicted by the theories. For KBr, a 1 mm thick layer can be considered as a semi-infinite layer. Comparison between existing theories of diffuse reflectance of powdered samples, shows that the statistical theories explain a good deal of the experimental observations concerning their variation with the particle parameters. This comparison shows a similarity between attenuated total reflection and diffuse reflectance spectra. Nevertheless, some discrepancies still persist; this may be due to the fact that the side-way scattering, which may be related to light transfer between neighbouring rough particles by frustrated total reflection, has only been indirectly taken into account.