Space-Resolved Chemical Information from Infrared Extinction Spectra

Abstract Based on the classical Lorentz model of the index of refraction, a new method is presented for the extraction of the complex index of refraction from the extinction efficiency, Qext , of homogeneous and layered dielectric spheres that simultaneously removes scattering effects and corrects measured extinction spectra for baseline shifts, tilts, curvature, and scaling. No reference spectrum is required and fit functions may be used that automatically satisfy the Kramers-Kronig relations. Thus, the method yields pure absorbance spectra for unambiguous interpretation of the chemical information of the sample. In the case of homogeneous spheres, the method also determines the radius of the sphere. In the case of layered spheres, the method determines the substances within each layer. Only a single-element detector is required. Using simulated Qext data of polymethyl-methacrylate and polystyrene homogeneous and layered spheres, we show that our reconstruction algorithm is reliable and accurately extracts pure absorbance spectra from Qext data. Reconstructing the pure absorbance spectrum from a published, experimentally measured raw absorbance spectrum shows that our method simultaneously corrects spectra for scattering effects and, given shape information, corrects raw spectra for systematic errors that result in spectral distortions such as baseline shifts, tilts, curvature, and scaling.