THE 217.5 nm BAND, INFRARED ABSORPTION, AND INFRARED EMISSION FEATURES IN HYDROGENATED AMORPHOUS CARBON NANOPARTICLES

Abstract

We report on the preparation of hydrogenated amorphous carbon nanoparticles whose spectral characteristics include an absorption band at 217.5 nm with the profile and characteristics of the interstellar 217.5 nm feature. Vibrational spectra of these particles also contain the features commonly observed in absorption and emission from dust in the diffuse interstellar medium. These materials are produced under "slow" deposition conditions by minimizing the flux of incident carbon atoms and by reducing surface mobility. The initial chemistry leads to the formation of carbon chains, together with a limited range of small aromatic ring molecules, and eventually results in carbon nanoparticles having an sp 2 /sp 3 ratio ≈ 0.4. Spectroscopic analysis of particle composition indicates that naphthalene and naphthalene derivatives are important constituents of this material. We suggest that carbon nanoparticles with similar composition are responsible for the appearance of the interstellar 217.5 nm band and outline how these particles can form in situ under diffuse cloud conditions by deposition of carbon on the surface of silicate grains. Spectral data from carbon nanoparticles formed under these conditions accurately reproduce IR emission spectra from a number of Galactic sources. We provide the first detailed fits to observational spectra of Type A and B emission sources based entirely on measured spectra of a carbonaceous material that can be produced in the laboratory.

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

“…Laboratory hydrogenated carbons have since then been considered realistic carriers of the bump (Colangeli et al 1997;Schnaiter et al 1998;Furton et al 1999;Dartois et al 2004). Laboratory a-C:Hs at different degrees of hydrogenation have been compared to observed astronomical spectra (Dartois et al 2007;Duley & Hu 2012). Soot nanoparticles prepared via laser pyrolisis (Jäger et al 2006(Jäger et al , 2008 are also candidates but only following the activation of π-π * transitions by UV irradiation (Gadallah et al 2011).…”

High-Resolution Fourier-Transform Microwave Spectroscopy of Methyl- And Dimethylnapthalenes

“…Alkyl substitution for hydrogen can also lead to non-zero dipole moments. Furthermore, laboratory-generated carbon nanoparticles containing alkyl-substituted naphthalenes have been shown to have infrared emission features consistent with dust in the ISM (Duley & Hu 2012), so alkylnaphthalenes are candidate species for identification in the ISM. Although rotationally resolved electronic spectra for 1-methylnapthalene, 2-methylnaphthalene (Tan et al 1991b), and 2,3-dimethylnaphthalene (Tan et al 1991a) have been reported in the past, no pure rotational spectra have previously been measured for alkylnaphthalenes.…”

An Organic Cosmo-Barometer: Distinct Pressure and Temperature Effects for Methyl Substituted Polycyclic Aromatic Hydrocarbons

“…Recently, specific dimethlynaphthalene isomers have been detected as constituents of experimentally produced hydrogenated amorphous carbon and compared to interstellar absorption features in carbonaceous dust in the diffuse interstellar medium and in circumstellar environments (Duley & Hu 2012). Moreover, the dimethylnaphthalenes are relatively easy to analyze by chromatographic and spectroscopic techniques and make ideal representatives of substituted aromatic hydrocarbons for a proof of concept study.…”