ON THE 2175-ANGSTROM ABSORPTION-BAND OF HOLLOW, ONION-LIKE CARBON PARTICLES

We present a theoretical result on the ultraviolet absorption of nano-scale graphite onions with a central void. The polarizability and the extinction cross section of particles in this geometry are obtained in the electrostatic (nonretarded) approximation and using a ''spherically uniaxial'' dielectric tensor transported from planar graphite. We study the influence of the ratio r/R of the inner and outer radii on the extinction efficiency. It appears that the onion particles for r/R almost-equal-to 0.6 well reproduce the observed UV absorption bump of interstellar dust around 5.7 eV (4.6 mum-1). We also investigate the effects of the hollow structure of the carbon particles on a second absorption peak around 18 eV.