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تسجيل مستخدم جديدEffects of structural and chemical disorders on the visible/UV spectra of carbonaceous interstellar grains
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The recent spectacular progress in the experimental and theoretical understanding of graphene, the basic constituent of graphite, is applied here to compute, from first principles, the UV extinction of nano-particles made of stacks of graphene layers. The theory also covers cases where graphene is affected by structural, chemical or orientation disorder, each disorder type being quantitatively defined by a single parameter. The extinction bumps carried by such model materials are found to have positions and widths falling in the same range as the known astronomical 2175 AA features: as the disorder parameter increases, the bump width increases from 0.85 to 2.5 $mu$m$^{-1}$, while its peak position shifts from 4.65 to 4.75 $mu$m$^{-1}$. Moderate degrees of disorder are enough to cover the range of widths of the vast majority of observed bumps (0.75 to 1.3 $mu$m$^{-1}$). Higher degrees account for outliers, also observed in the sky. The introduction of structural or chemical disorder amounts to changing the initial $sp^{2}$ bondings into $sp^{3}$ or $sp^{1}$, so the optical properties of the model material become similar to those of the more or less amorphous carbon-rich materials studied in the laboratory: a-C, a-C:H, HAC, ACH, coals etc. The present treatment thus bridges gaps between physically different model materials.
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