Carbon is one of the main constituent elements of interstellar dust. Due to their strong absorptivity and emissivity in a wide wavelength region, carbon nano-particles play an important role in the heat budget of interstellar clouds and are directly involved in the process of star formation. Because carbon nano-particles may provide new unique physical properties they also attain great importance in modern material research.
The ubiquitous interstellar extinction curve reflects the scattering and absorption characteristics of the grains embedded in the diffuse interstellar medium. Therefore, it contains information of the grain size as well as the particle material. In the ultraviolet (UV) the extinction curve is dominated by a strong absorption feature - the 217.5 nm bump. In the past, based on scattering calculations, spherical nano-sized graphite particles are by far the most proposed candidate for this feature. Today this assignment gets more and more questionable, for the following reasons:
Graphite is an anisotropic material and therefore can hardly be found in spherical form in nature. A ``fine-tuning'' of the bulk optical constants is required to fit the feature. The peak position of the bump predicted for graphite particles is quite sensitive to grain size, shape, and coatings, which is inconsistent with the observations. The internal structure of earth synthesised and meteoritic carbon particles are characterized by a great diversity and are designated as amorphous, disordered, or paracrystalline.
The probably best identification of the 217.5 nm feature would be an electronic pi-pi* transition in a nano-sized carbonaceous material which is produced under conditions relevant to astrophysics and the structure of which is entirely charcterized. However, the investigations performed so far on carbon nano-grains suffered from the fact that the extinction properties were determined on heavily agglomerated particle samples. It has been long suggested that grain agglomeration has a strong influence on the spectral appearance and could account for the broadening of the UV bump measured on such samples with respect to the 217.5 nm feature. Therefore, the possibility to measure the extinction of non-clustered carbon nano-particles has been claimed for a long time but failed so far in its experimental conversion.