Obtaining the complex optical constants n and k via quantitative absorption measurements in KBr pellets

摘要

Reflectance and emittance spectroscopy, especially at infrared wavelengths, continues to grow in utility as an analyticaltechnique for contact, standoff, and remote sensing. The reflectance spectra of solids, however, are complex, dependingon many parameters, even for the same material. Granule or powder particle size, crystal morphology, layer thickness,and substrate material all affect the spectral distribution of reflected light. However, such phenomena can all be modeledif the optical constants n(ν) and k(ν) are available. If the quantitative absorption coefficient K(ν) is known, the k value canbe obtained via the relation k(ν) = 2.303K(ν)/4πν. The absorption coefficient can in turn be derived from a simple Kbrpelletinfrared absorption measurement, provided the pellet mass ratio is prepared quantitatively. The method requires thepellet’s mass and diameter, along with the analyte mass fraction and density. In this paper we demonstrate the requisiteexperimental details in preparing the pellets, as well as methods to reduce light scattering in order to obtain morequantitative values. Theoretical methods to derive the related optical constants will also be detailed, in particular theassumptions used to obtain the scalar refractive index n. Ideally, this value is known or measured separately, but in somecases we have found that it can be approximated (first approximation) for most organic chemicals by n～1.5 at the shortestwavelength. The method and results are presented for two different species, aspartame and rhodamine B.