Hydrocarbon nanoparticles as a diffuse ISM analogue: morphology and infrared absorption in the 2000-500 cm-1 region

In this paper, we analyse the morphology and chemistry of carbonaceous dust particles previously proposed as a candidate analogue material for dust in the diffuse interstellar medium (ISM). The particles were polymerized in a low pressure capacitively coupled radiofrequency discharge in mixtures of argon/acetylene and helium/acetylene. Infrared (IR) spectra of our dust particles reveal the strong presence of aliphatic features, both stretching and bending modes, which are very similar to astronomical data from diffuse interstellar dust. A comparison between the IR spectra obtained using argon or helium as buffer gases reveals the strong presence of C = C double bonds (skeletal modes) when helium is used. The presence of these skeletal modes is clearly visible through the broad absorption feature at 1600 cm-1 (6.25 μm). The origin of this feature in our laboratory analogue is related to the change in the plasma parameters due to the variation of the buffer gas used under the same discharge conditions. Observational data of interstellar polycyclic aromatic hydrocarbons also exhibit a feature at this wavelength