Carbonaceous onion-like particles as a component of interstellar dust

The search for the carrier of the 217.5 nm absorption feature in the inters tellar extinction curve has been undertaken for the past few decades. We ha ve previously discussed a carbonaceous material named quenched carbonaceous composite (QCC) as a laboratory analog for the interstellar material produ cing this absorption feature. QCC is condensed from an ejecta of hydrocarbo n plasma. High-resolution transmission electron micrographs reveal that QCC is a coagulation of carbonaceous onion-like particles. Each particle consi sts of multiple shells, and many of the particles have a void at the center . By X-ray photoelectron spectroscopy, the ratio of sp(3)-hybridized carbon to sp(2)-hybridized carbon in the QCC is estimated to be in the range of 0 .16-0.4. This is much lower than other types of materials such as amorphous carbon and hydrogenated amorphous carbon. The 220 nm absorbance peak of th e QCC is not stable against heating, suggesting that either a volatile comp onent gives rise to the 220 nm absorption or structural changes occur withi n the sample. Our experiments indicate that carbonaceous onion-like dust pa rticles are a possible carrier of the 217 nm interstellar medium absorption .