Plasma generated analogs of interstellar carbonaceous dust: Theoretical modeling and electron bombardment

Abstract

Observational evidence, based on IR absorption spectra, indicates that interstellar (IS) carbonaceous dust in the diffuse interstellar medium, is largely made of some sort of amorphous hydrogenated carbon (abbreviated HAC or a¿C:H). At present, two alternative models can be found in the literature. One of them favors a solid, made by small aromatic islands linked by aliphatic chains [1], whereas the other proposes large polyaromatic structures with small aliphatic substituents at the edges [2]. Theoretical models of amorphous HAC solids, based on the mentioned competing structures [1] [2b], were constructed by our group using Density Functional Theory (DFT), and their calculated IR spectra were compared to those of HAC samples generated by plasma enhanced chemical vapor deposition (PECVD). The deposits were grown in inductively coupled RF plasmas of CH4/He mixtures. The deposition conditions were selected to obtain HAC films with a variable proportion of aliphatic and aromatic structures. The results suggest that the estimation of the hydrogen contents and the aliphatic/aromatic ratios of HAC samples using just literature IR band strengths advanced by some authors[2a], is difficult and might be not reliable. Observations also show, that the most prominent spectroscopic feature associated with the aliphatic component of IS carbonaceous dust ( the 3.4 um absorption band ) disappears in dense molecular clouds , although the details of the destruction mechanism are not entirely clear. The effects of cosmic rays, which can reach the interior of dense clouds, on the carriers of this feature were investigated by irradiating plasma¿ generated HAC samples with 5 keV electrons. The estimated effects of cosmic rays are found to be small and are not enough to explain the disappearance of the 3.4 um band.