Near-infrared transmission spectroscopy can be complicated by the light scattering from heterogeneous materials. For the examination of an evolving system exhibiting such light scattering, transmission spectra near wavenumber υ = 104 cm-1 were obtained during the hydrolysis of FeCl3 solutions. At first, the resulting turbid suspension of cigar-shaped β-FeOOH particles exhibits single-particle scattering, including a Rayleigh regime (attenuation ∝ υ4). At later times, the scattering increases strongly as the particles aggregate, and becomes proportional to υα, with α ≈ 2, consistent with scattering models that interpret the structure of aggregates in terms of a fractal dimension df roughly equal to 2. In all cases investigated, the attenuation due to scattering is spectrally smooth and increases monotonically with wavenumber. It can be written in the simple form υα with 1≤ α ≤ 4. While over limited spectral ranges α may be taken independent of υ , over wide ranges it decreases with increasing υ. This behavior is consistent with the theoretical limits of α = 4 at υ = 0, and α = 0 at υ = ∞. Overall, the results suggest that a useful form for simulating scattering backgrounds in near-infrared spectroscopy is Aυα, with A and α fitted constants.
P. Berdahl, L. H. Espinoza, D. Littlejohn, D. Lucas, and D. L. Perry, "Near-Infrared Turbidity of β-FeOOH Particle Suspensions," Appl. Spectrosc. 54, 262-267 (2000)
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