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

Abstract

Context. 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. Aims. We study the spectral properties of carbonaceous dust analogs from the far-ultraviolet to the mid-infrared and correlate features in both spectral ranges to the aromatic/aliphatic degree. Methods. Analogs to carbonaceous interstellar dust encountered in various phases of the interstellar medium have been prepared in the laboratory. These are amorphous hydrogenated carbons (a-C:H), analogs to the diffuse interstellar medium component, and soot particles, analogs to the polyaromatic component. Thin films (d < 100 nm) have been measured in transmission in the vacuumultraviolet (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 µm) and in the mid-infrared (3-15 µm). Results. Tauc optical gaps, E g , are derived from the visible spectra. The major spectral features are fitted through the VUV to the midinfrared to obtain positions, full-widths at half maximum (FWHM), and integrated intensities. These are plotted against the position of the π-π * 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 π-π * electronic transitions peak is found. The latter is also correlated to the position of the sp 3 carbon defect band at ∼8 µm, the aromatic C=C stretching mode position at ∼6 µm, and the H/C ratio. Conclusions. Ultraviolet and infrared spectroscopy of structurally diverse carbon samples are used to constrain the nanostructural properties of carbon carriers of both circumstellar and interstellar extinction, such as the associated coherent lengths and the size of polyaromatic units. 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.

Quantum modeling of the optical spectra of carbon cluster structural families and relation to the interstellar extinction UV bump

“…More generally, the contribution of more disordered species was further elaborated under the form of hydrogenated amorphous carbon (HAC; Jones et al 1990;Gadallah et al 2011) and soot particles (Wada et al 1999;Gavilan et al 2016Gavilan et al , 2017. In an attempt to establish correlations between carbon grain morphology and spectral data, Rotundi et al (1998) proposed that carbon nanostructures ordered on the micrometer scale could be better candidates to interpret the UV bump rather than graphitic particles.…”

“…In an attempt to establish correlations between carbon grain morphology and spectral data, Rotundi et al (1998) proposed that carbon nanostructures ordered on the micrometer scale could be better candidates to interpret the UV bump rather than graphitic particles. This motivated the determination of experimental optical spectra of soot particles for species produced either in plasma form (Wada et al 1999) or in flames (Gavilan et al 2016(Gavilan et al , 2017. Wada et al (1999) produced what they refer to as quenched carbonaceous composites (QCCs), and proposed that they were constituted of onion-like carbonaceous particles with a small amount of sp 3 carbon atoms.…”

“…Wada et al (1999) produced what they refer to as quenched carbonaceous composites (QCCs), and proposed that they were constituted of onion-like carbonaceous particles with a small amount of sp 3 carbon atoms. In their flame experiments, Gavilan et al (2017) produced compounds analogous to interstellar carbon dust with various levels of nanostructuration (Carpentier et al 2012), leading to variations in the positions of the UV bump.…”

Carbon ‘fluffy’ aggregates produced by helium–hydrocarbon high-pressure plasmas as analogues to interstellar dust

“…In order to mimic the chemical and typological features of interplanetary and interstellar carbon dust, many synthesis methods have been proposed based on condensation experiments JÃd'ger et al 2009), physical vapor deposition (Dartois et al 2005), combustion and pyrolysis methods (Colangeli et al 1997;Pino et al 2008;Biennier et al 2009;Carpentier et al 2012;Gadallah et al 2013;Gavilan et al 2016Gavilan et al , 2017, pulsed laser deposition (Mennella et al 2002b;JÃd'ger et al 2008;Godard et al 2011;Gadallah et al 2012Gadallah et al , 2013 and plasma enhanced chemical vapor deposition (Colangeli et al 1997;Kovačević et al 2005;Stefanović et al 2005Stefanović et al , 2007Maté et al 2016;Molpeceres et al 2017).…”

“…Moreover, the plasma synthesized dust analogs can be used in laboratory studies concerning the effects of different physical agents on structural properties, in order to mimic astrophysical processes or observed spectral features. For example, the exposure of hydrogenated amorphous carbon samples to 160 nm UV photons, at doses comparable with the average doses in the diffuse interstellar medium, was carried out in order to obtain information on spectroscopic features of dust in diffuse interstellar medium such as the UV bump at 217 nm (4.6 µm -1 ) (Gadallah et al 2011;Gavilan et al 2016) or the mid-far infrared bands (Mennella et al 2001b;Gadallah et al 2012;Maté et al 2014;Gavilan et al 2017). Moreover, exposure of hydrogenated amorphous carbon samples to an electron beam of 5 keV energy, was found to be an approach in studying the de-cay of the 3.4 µm band, characteristic to destruction of CH 3 and CH 2 groups in dense clouds by cosmic rays (Maté et al 2014(Maté et al , 2016.…”