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 = 10<sup>4</sup> cm<sup>-1</sup> were obtained during the hydrolysis of FeCl<sub>3</sub> solutions. At first, the resulting turbid suspension of cigar-shaped β-FeOOH particles exhibits single-particle scattering, including a Rayleigh regime (attenuation ∝ υ<sup>4</sup>). At later times, the scattering increases strongly as the particles aggregate, and becomes proportional to υ<sup>α</sup>, with α ≈ 2, consistent with scattering models that interpret the structure of aggregates in terms of a fractal dimension d<sub>f</sub> 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 υ<sup>α</sup> 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 <i>Aυ</i><sup>α</sup>, 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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