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Near-Infrared Turbidity of beta-FeOOH Particle Suspensions

Volume 54, Number 2 (Feb. 2000) Page 262-267

Berdahl, P.; Espinoza, L.H.; Littlejohn, D.; Lucas, D.; Perry, D.L.

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 upsilon = 104 cm-1 were obtained during the hydrolysis of FeCl3 solutions. At first, the resulting turbid suspension of cigar-shaped beta-FeOOH particles exhibits single-particle scattering, including a Rayleigh regime (attenuation upsilon4). At later times, the scattering increases strongly as the particles aggregate, and becomes proportional to upsilonalpha, with alpha 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 upsilonalpha with 1 < = alpha < = 4. While over limited spectral ranges alpha may be taken independent of upsilon , over wide ranges it decreases with increasing upsilon. This behavior is consistent with the theoretical limits of alpha = 4 at upsilon = 0, and alpha = 0 at upsilon = Infinity. Overall, the results suggest that a useful form for simulating scattering backgrounds in near-infrared spectroscopy is A upsilonalpha, with A and alpha fitted constants.