Characterization of interstellar carbon dust analogues synthesized by dielectric barrier discharge and evolution after irradiation with 3 MeV H+

ABSTRACT ‘Fluffy’ hydrogenated amorphous carbon (a-C:H) was synthesized using a dielectric barrier discharge plasma, driven by nanosecond high voltage pulses at 1 kHz frequency in a helium–butane mixture. The a-C:H samples were characterized by scanning and transmission electron microscopy, laser-assisted and secondary ion mass spectrometry, and Raman and Fourier-transform infrared spectroscopy. We find that a-C:H samples exhibit infrared absorption features in good agreement with those observed for carbonaceous dust in IRAS 08572 + 3915 galaxy. We discuss their nano- to microscale structure and derive their hydrogen to carbon (H/C) ratios from the results obtained by three distinct experimental characterization techniques. Relying on the average H/C value determined by mass spectrometry and Raman spectroscopy, we can then constrain the absorption strengths values to those best corresponding to our dust analogue, and calculate the H/C ratio from the infrared spectra. Altogether, we find that our dust analogue consists of a dominant hydrogen-rich aliphatic network, with small, isolated, aromatic regions. The a-C:H dust analogue was then irradiated with 3 MeV H+ and subsequently analysed ex situ. Morphological and chemical changes, including the evolution of H/C, CH2/CH3, and sp2/sp3 ratios, were observed with increasing proton fluence, indicating dehydrogenation and graphitization. Proton bombardment shifted the initial location of a-C:H in the hydrocarbon ternary phase diagram toward the central region defined by IRAS 08572 + 3915 observations. The decay of the 3.4 $\mu$m band with proton fluence was used to calculate CH destruction cross-sections, results consistent with a direct effect of cosmic rays on the disappearance of the 3.4 $\mu$m band.