Analogs of solid nanoparticles as precursors of aromatic hydrocarbons

Abstract

Context. Aromatic =CH and C=C vibrational bands have been observed within shocked interstellar regions, indicating the presence of aromatic emission carriers such as PAHs, which may have been created from adjacent molecular cloud material by interaction with a shock front. Aims. We investigate the evolution of the aromatic =CH and C=C vibrational modes at 3.3 and 6.2 μm wavelength in heated HAC materials, PAHs and mixed PAHs and HACs, respectively, aiming at an explanation of the evolution of carbonaceous dust grains in the shocked regions. Methods. Materials used in these analogs (HAC and PAH materials) were prepared by the laser ablation and the laser pyrolysis methods, respectively. The transmission electron microscopy (TEM) in high-resolution mode was used as an analytical technique to characterize the aromatic layers in HACs. Spectroscopic analysis was prformed in the mid-IR range. Results. A remarkable destruction of aliphatic structures in HACs has been observed with the thermal processing, while aromatic structures become dominating by increasing the diameters of the graphene layers. The aromatic bands at 3.3 and 6.2 μm, observed in the laboratory spectra of PAHs and of the combination of the PAHs and HAC materials, are also clearly observed in the spectrum of the heated HACs. These bands agree with those of aromatic bands observed in astronomical observations. Conclusions. Aromatization of HACs could be a pre-stage in the decomposition process of hydrocarbons that form PAH-clusters in such hot interstellar medium.

A search for hydrogenated fullerenes in fullerene-containing planetary nebulae

“…However, their anharmonic DFT calculations work well for small PAHs in this spectral region, opening the way to predict the spectrum of larger PAHs (and other hydrocarbons). Finally, Gadallah et al (2013) also found that the 3.3 µm and 6.2 µm AIBs are also clearly observed in the spectrum of heated HAC dust.…”

“…Fullerenes and fullerene-based molecules, such as hydrogenated fullerenes and buckyonions, may explain some astrophysical phenomena, like the intense UV absorption band at 217 nm (e.g., Cataldo & Iglesias-Groth 2009) or the so-called diffuse interstellar bands (DIBs; see e.g., Cox 2011;Iglesias-Groth 2007;García-Hernández & Díaz-Luis 2013;Díaz-Luis et al 2015). Owing to the remarkable stability of fullerenes against intense radiation and ionization, these molecules are good candidates for being widespread in insterstellar/circumstellar media.…”

Dust in brown dwarfs and extrasolar planets

“…. 3.6µm were observed in the ISM and interpreted as a mix of HAC and PAH absorption representing small dust grains (Gadallah et al 2013). The transmission spectrum of the hot Jupiter WASP-12b is almost completely flat.…”

Vacuum ultraviolet photolysis of hydrogenated amorphous carbons

“…By number, class A spectra are the most often observed, followed by class B, and a few class C. The evolution among the profiles is primarily linked to differences in the chemical composition of the emitting molecules or particles. Class A are probably the most aromatic ones, and class C display additional features that are attributed to a more aliphatic character, observationnally (Sloan et al 2007;Boersma et al 2008;Keller et al 2008;Acke et al 2010), and experimentally (Pino et al 2008;Carpentier et al 2012;Gadallah et al 2013).…”