On the optics of small graphite spheres. II.

Abstract

Mie computations are performed for small graphite spheres using the model refractive index given in Paper I. The efficiency factor Q_ext and the albedo γ are computed as functions of wavelength for particles of radii between 0.010 µ and 0.065µ. For radii less than ~0.03µ the extinction follows an approximately λ ^-1 law in the range x.3µ > λ> 0.45 µ, but begins to rise more steeply for shorter wavelengths. Normalized extinction curves are constructed for Gaussian distributions of particle sizes with small dispersions centred at a̅ = 0.015 µ 0.020µ, 0.025µ. Agreement with the observational curves remains generally tolerable for wavelengths longward of 4500 Å, but the theoretical curves show excess extinction towards the ultra-violet. For a grain radius a= 0.02µ the ratio Q_ext (2000Å)/Q_ext(4000Å), ≌ 7, but this ratio decreases with increasing radius. Good agreement with the observational curve is obtained for an Oort-van de Hulst distribution of graphite grains with a mean size a̅ = 0.028µ.