Carbonaceous compounds are found in very diverse astronomical media [1], a significant amount consisting in small dust grains. This dust originates characteristic IR absorption bands revealing the presence of aliphatic and aromatic functional groups in variable proportions [2, 3]. Notably, the characteristic 3.4 ¿m (2940 cm-1) band is observed in the diffuse interstellar media, but not in dense clouds, and is mostly attributed to CH stretching vibrations of aliphatic groups (CH2, CH3). Among the various materials investigated as possible carriers of these bands, plasma generated hydrogenated amorphous carbon (a-C:H) led to the best agreement with observations [2-4], although the exact composition and structure are still not clear and under intensive investigation. In this work we present a study of the structure and spectroscopic features of carbonaceous dust analogues generated in He + CH4 inductively coupled RF discharges. Spectra obtained from density functional theory (DFT) calculations of different hydrocarbon structures are compared to the IR spectra of the samples to identify the structures that better fit the experimental results. The effects of cosmic rays (CRs) on the 3.4 ¿m IR band of a-C:H are also simulated by bombarding the samples with a beam of 5 keV electrons, which impart energy doses comparable to those of MeV protons [5].

The authors acknowledge financial support from the Spanish MINECO through grants SD2009-00038 (Consolider Astromol), FIS2013- 48087-C2-1-P and, FIS2016‐77726‐C3‐1‐P. Additional funding from ERC-2013-Syg 610256-NANOCOSMOS is also acknowledge

Santiago de Compostela, Facultade de Química,17-21 julio 2017. -- http://www.bienalrsef2017.com/bienalrsef17/

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