A polycrystalline graphite model for the 2175 Å interstellar extinction band

A random, hydrogen-free, assembly of microscopic sp2 carbon chips, forming a macroscopically homogeneous and isotropic solid, is proposed as a model carrier for the ultraviolet (UV) interstellar extinction band. The validity of this model is based on the calculation of the Bruggeman average dielectric function of a mixture of the known parallel and perpendicular dielectric functions of graphite. The π absorption feature of Rayleigh-sized spheres of this mixture falls near 4.6 μm−1 (2175 Å), but its width is 1.5 μm−1, somewhat larger than the astronomically observed average, 1 μm−1. This is confirmed by measurements of the reflectance of an industrial material, polycrystalline graphite. A better fit to the interstellar feature position and width is obtained with a hypothetical material, having the same dielectric functions as natural graphite, except for less extended wings of the π resonance. Physically, this could result from changes in the electronic band structure due to previous thermal histories. In this model, the Frölich feature central wavelength depends only on the π resonance frequency, while its width depends only on the damping constant of the same resonance. This explains the range of observed feature widths at constant feature wavelength.