Polyaromatic disordered carbon grains as carriers of the UV bump: FUV to mid-infrared spectroscopy of laboratory analogs

Abstract

A multiwavelength study of laboratory carbons with varying degrees of hydrogenation and sp${}^2$ hybridization is required to characterize the structure of the carbonaceous carriers of interstellar and circumstellar extinction. Analogs to carbonaceous interstellar dust encountered in various phases of the interstellar medium have been prepared in the laboratory. Thin films have been measured in transmission in the vacuum ultraviolet (VUV; 120 - 210 nm) within the atmospheric pressure experiment (APEX) chamber of the DISCO beam line at the SOLEIL synchrotron radiation facility. Spectra of these films were further measured through the UV-Vis (210 nm - 1 $\mu$m) and in the mid-infrared (3 - 15 $\mu$m). Tauc optical gaps, E${}_g$, are derived from the visible spectra. The major spectral features are fitted through the VUV to the mid infrared to obtain positions, full-widths at half maximum (FWHM), and integrated intensities. These are plotted against the position of the $\pi$-${\pi }^{\ast }$ electronic transitions peak. Unidentified or overlapping features in the UV are identified by correlations with complementary infrared data. A correlation between the optical gap and position of the $\pi$-${\pi }^{\ast }$ electronic transitions peak is found. The latter is also correlated to the position of the sp${}^3$ carbon defect band at ~8 $\mu$m, the aromatic C=C stretching mode position at ~6 $\mu$m, and the H/C ratio. Our study suggests that carriers of the interstellar UV bump should exhibit infrared bands akin to the A/B classes of the aromatic infrared bands, while the circumstellar bump carriers should exhibit bands corresponding to the B/C classes.

arXiv comment20 pages, 101 figures. Accepted for publication in Astronomy & Astrophysics (24 July 2017)

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• Carbon 'fluffy' aggregates produced by helium - hydrocarbon high pressure plasmas as analogs to interstellar dust
• Dust changes in Sakurai's Object: new PAHs and SiC with coagulation of submicron-sized silicate dust into 10 micron-sized melilite grains
• Hydrogenated amorphous carbon grains as an alternative carrier of the 9$-$13$\mu$m plateau feature in the fullerene planetary nebula Tc 1
• Finite-temperature stability of hydrocarbons: fullerenes versus flakes