Matrix-Isolated Nano-Sized Soot Grains and Their Relation to Solid Carbon in Space

Abstract

The effect of particle shape or particle clustering onto the extinction behavior of nano-sized hydrogenated carbon grains was investigated experimentally. The particles were extracted by a molecular beam technique at different condensation and clustering states and were isolated in an argon matrix for UV spectroscopy. The state of clustering in the samples was controlled by TEM analysis. A clear correlation was found between the measured UV feature width and the degree of particle clustering in agreement with theoretical investigations for compact clusters. Therefore, the results show that the optical properties of carbonaceous grain material are strongly influenced by the particle shape and the clustering degree. For particles produced in hydrogen-containing atmospheres, the UV extinction peak was shifted blueward to a position close to the position of the 217.5 nm hump. This shift was found to be nearly independent of the amount of hydrogen in the condensation zone. As in the case of the interstellar 217.5 nm hump the UV features of the matrix isolated soot grains can fitted very well by a “Drude” profile plus a ∼ λ⁻¹ background. Our experimental results show the most narrow UV profiles ever measured on carbonaceous dust analogues in the laboratory and are comparable to the width of the interstellar 217.5 nm hump.