VUV spectroscopy of carbon dust analogs: contribution to interstellar extinction

Abstract

Context.A full spectral characterization of carbonaceous dust analogs is necessary to understand their potential as carriers of observed astronomical spectral signatures such as the ubiquitous UV bump at 217.5 nm and the far-ultraviolet (FUV) rise common to interstellar extinction curves. Aims. Our goal is to study the spectral properties of carbonaceous dust analogs from the FUV to the mid-infrared (MIR) domain. We seek in particular to understand the spectra of these materials in the FUV range, for which laboratory studies are scarce. Methods. We produced analogs to carbonaceous interstellar dust encountered in various phases of the interstellar medium: amorphous hydrogenated carbons (a-C:H), for carbonaceous dust observed in the diffuse interstellar medium, and soot particles, for the polyaromatic component. Analogs to a-C:H dust were produced using a radio-frequency plasma reactor at low pressures, and soot nanoparticles films were produced in an ethylene (C 2 H 4 ) flame. We measured transmission spectra of these thin films (thickness <100 nm) in the far-ultraviolet (190−250 nm) and in the vacuum-ultraviolet (VUV; 50−190 nm) regions using the APEX chamber at the DISCO beam line of the SOLEIL synchrotron radiation facility. These were also characterized through infrared microscopy at the SMIS beam line. Results. We successfully measured the transmission spectra of these analogs from λ = 1 μm to 50 nm. From these, we extracted the laboratory optical constants via Kramers-Kronig inversion. We used these constants for comparison to existing interstellar extinction curves. Conclusions. We extend the spectral measurements of these types of carbonaceous analogs into the VUV and link the spectral features in this range to the 3.4 μm band. We suggest that these two materials might contribute to different classes of interstellar extinction curves.

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

“…3.4. For the a-C:H prepared with a toluene precursor, the optical gap was found to be 1.95 eV, lower than the 2.5 eV estimated for the methane-precursor a-C:H of Gavilan et al (2016). For the soot samples produced with the C/O = 0.82 flame, E g decreases from 0.78 to 0.62 eV as the HAB increases.…”

“…A direct comparison of the FWHM of a laboratory soot (1.88 µm −1 ) to the newly parametrized FWHM (1.78 µm −1 ) from the average interstellar extinction curve indicates that laboratory polyaromatic solid carbons can approximate the average carrier of the UV bump. However, it is more likely than more than one kind of carbon is measured towards a line of sight, and that more hydrogenated carbons contribute strongly to the FUV rise as shown in Gavilan et al (2016) and in Fig. 14, where our laboratory soot and a-C:H spectra are compared to extinction curves in the ISM and CSM.…”

“…Few laboratory spectroscopic studies on carbonaceous dust extending over a wide spectral range exist, due to experimental difficulties in accessing the vacuum-ultraviolet (VUV). In this range, different techniques have been used including electron energy loss spectroscopy (EELS; Fink et al 1984;Zubko et al 1996) and synchrotron VUV sources (Colangeli et al 1993;Gavilan et al 2016).…”

The global dust modelling framework THEMIS

“…There is a large database of laboratory-measured (e.g., Smith 1984;Duley 1984;Mennella et al 1995;Gavilan et al 2016) 5 and post-processed (e.g., Rouleau & Martin 1991;Zubko et al 1996) 6 optical property data for a-C(:H) materials but it is far from homogeneous in terms of composition/structure, synthesis methods and wavelength coverage. Hence, until such time as the gaps have been filled we rely upon a recentlydeveloped set of optical properties for a large part of the a-C(:H) family parameter space (Jones 2012a,b,c).…”

Swift heavy ion irradiation of interstellar dust analogues

“…This was estimated from the imaginary part of the refractive index (k) measured for the same analogues in the VUV (Gavilan et al 2016), using the CH photolysis cross section of σ ≈ 3 ± 0.9 × 10 −19 cm 2 from Alata et al (2014) for a broader VUV photon range emitted by an H 2 plasma discharge lamp. We fit the new set of ion cross section data with a power law,…”