Astronomical observations have revealed the presence of a ubiquitous absroption band at 3.4 ¿m inthe diffuse interstellar (IS) medium. It is generally assumed that this band corresponds to some sort of amorphous hydrogenated carbon (a¿C:H), whose structure has been explained by two alternative models: small aromatic islands linked by aliphatic chains [1] or large polyaromatic structures with small aliphatic substituents at the edges [2]. In addition, it has been found that the 3.4 ¿m absorption band related to the aliphatic component of IS carbonaceous dust disappears inside dense molecular clouds. In this environment, cosmic rays (CR) could provide a realistic destruction mechanism. In order to clarify these issues, IS carbonaceous dust analogs produced in cold hydrocarbon plasmas and processed by highly energetic electrons has been analyzed by IR absorption, and the spectral signatures have been related with chemical structures described theoretically. Thin films and dust grains of a¿C:H with a variable proportion of aliphatic and aromatic structures are produced by plasma deposition in RF discharges using mixtures of hydrocarbons and He as plasma precursors. Optical spectroscopy, mass spectrometry and Langmuir probes are used for plasma diagnostics. The a¿C:H deposits are analyzed with IR absorption spectroscopy and surface analysis techniques (SEM, AFM,...). The effects of cosmic rays are investigated by irradiating the samples with 5 keV electrons. Models of a¿C:H solids of variable density, based on the mentioned competing structures [1,2], are constructed using Montecarlo/Molecular Mechanics and their electronic energies and IR spectra are computed at Density Functional Theory (DFT) level. Comparison between theoretical and experimental IR spectra [3] shows that the structure of carbonaceous dust in the diffuse IS medium is most probably intermediate between those of the two literature models but closer to that with small aromatic units [1]. Electron bombardment, which is used as a substitute for cosmic ray irradiation, induces dehydrogenation and aromatization of the carbonaceous solids but the experimental destruction rates indicate that the effects of cosmic rays are not enough to explain the disappearance of the aliphatic dust component,characterized by the 3.4 ¿m band, inside dense clouds [4].

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 acknowledged

RIVA-X, Bilbao, 4-6 October 2017. -- http://asevaconferences.com/riva-x/

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