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

TL;DR

This study synthesizes hydrogenated amorphous carbon dust analogues using plasma methods, characterizes their structure and IR features, and investigates their chemical evolution under proton irradiation to understand cosmic ray effects on interstellar dust.

Contribution

It provides a detailed characterization of a-C:H dust analogues and explores their chemical and morphological changes after proton irradiation, simulating cosmic ray interactions.

Findings

a-C:H analogues exhibit IR features similar to interstellar dust

Proton irradiation causes dehydrogenation and graphitization of the dust analogues

Decay of the 3.4 μm band allows estimation of CH destruction cross-sections

Abstract

"Fluffy" hydrogenated amorphouscarbon(a-C:H)wassynthesizedusingadielectric barrier discharge plasma, driven by nanosec ond 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 analyzed 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.