The experimentally determined extinction spectra of colloidal hematite, consisting of spherical particles of 0.10,0.12,0.13,0.15, 0.16, and 0.51 µm in modal diameter, respectively, are compared with the corresponding theoretically calculated spectra using the Lorenz-Mie theory. These spectra are most sensitive to the particle size in the 0.10–0.20-µm range due to the damping effects which broaden the resonance at the expense of its height. For large particles (0.51 µm) the extinction becomes independent of the wavelength because of the broader size distribution of the hematite dispersion. As the small particles aggregate, a shoulder appears at 550 nm corresponding to the onset in fluctuations in the extinction efficiency qf various sized particles. This shoulder may suggest the existence of certain surface electromagnetic modes caused by aggregation. A comparison is made in the sizedependent extinction efficiency at various wavelengths between colloidal hematite and gold sols.

© 1985 Optical Society of America

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