A Candidate Analog for Carbonaceous Interstellar Dust: Formation by Reactive Plasma Polymerization

Carbonaceous compounds are a significant component of interstellar dust, and the composition and structure of such materials is therefore of key importance. We present 1.5-15 μm spectra of a plasma-polymerized carbonaceous material produced in radio-frequency discharge under low pressure, using C₂H₂ as a precursor component. The infrared spectra of the resulting spheroidal carbonaceous nanoparticles reveal a strong aliphatic band (3.4 μm feature), weak OH and carbonyl bands, and traces of aromatic compounds, all characteristics identified with dust in the diffuse interstellar medium of our Galaxy. The plasma polymerization process described here provides a convenient way to make carbonaceous interstellar dust analogs under controlled conditions and to compare their characteristics with astronomical observations. Here we focus on a comparison with the IR spectra of interstellar dust. The IR spectrum of carbonaceous dust in the diffuse interstellar medium is characterized by a strong 3.4 μm C–H stretching band and weak 6.8 and 7.2 μm C–H bending bands, with little evidence for the presence of oxygen in the form of carbonyl (C=O) or hydroxide (OH) groups. The plasma polymerization products produced under oxygen-poor conditions compare well with the peak position and profiles of the observed IR spectrum of diffuse dust. In addition, we find that addition of nitrogen to the plasma results in bands at 6.15 μm (C=N band) and at 3 μm (NH band). We note that, with the addition of nitrogen, the 3.4 μm hydrocarbon band diminishes greatly in strength as the NH band grows. This may have implications for the puzzling absence of the 3.4 μm hydrocarbon bands in the IR spectra of dust in dense molecular clouds, given that the presence of nitrogen-related bands has been established in dense-cloud dust.