Abstract
The frequency-dependent extinction cross section of an ellipsoidal particle is examined from the point of view of sum rules and optical moments. For particles of any size a second-moment representation generates a characteristic extinction cross-section frequency that depends only on the plasma frequency, the dc dielectric constant of the bulk material, and the particle shape and is independent of the order or the disorder inside the object. In addition, this characteristic extinction frequency has the same value as the characteristic absorption frequency previously found for the small-particle Rayleigh limit. To illustrate these intrinsic features, numerical calculations are carried out on spherical shells and on silicon spheres. A spherical particle with fractal structure is used to demonstrate that the characteristic frequency can depend on a special internal structure of the particle. Finally, the characteristic frequency of a dust of randomly oriented ellipsoids is determined. Since the characteristic extinction frequency is independent of particle size, size distributions cannot influence its value.
© 1996 Optical Society of America
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