Far- and mid-infrared spectroscopy of complex organic matter of astrochemical interest: coal, heavy petroleum fractions and asphaltenes

Abstract

The coexistence of a large variety of molecular species (i.e. aromatic, cycloaliphatic and aliphatic) in several astrophysical environments suggests that unidentified infrared emission (UIE) occurs from small solid particles containing a mix of aromatic and aliphatic structures (e.g. coal, petroleum, etc.), renewing the astronomical interest on this type of materials. A series of heavy petroleum fractions namely ‘distillate aromatic extract’, ‘Residual Aromatic Extract’, heavy aromatic fraction (BQ-1) and asphaltenes derived from BQ-1 were used together with anthracite coal and bitumen as model compounds in matching the band pattern of the emission features of proto-planetary nebulae (PPNe). All the model materials were examined in the mid-infrared (2.5–16.66 μm) and for the first time in the far-infrared (16.66–200 μm), and the infrared bands were compared with the UIE from PPNe. The best match of the PPNe band pattern is offered by the BQ-1 heavy aromatic oil fraction and by its asphaltenes fraction. Particularly interesting is the ability of BQ-1 to match the band pattern of the aromatic–aliphatic C–H stretching bands of certain PPNe, a result which is not achieved neither by the coal model nor by the other petroleum fractions considered here. This study shows that a new interesting molecular model of the emission features of PPNe is asphaltene molecules which are composed by an aromatic core containing three to four condensed aromatic rings surrounded by cycloaliphatic (naphtenic) and aliphatic alkyl chains. Instead, the weakness of the model involving a mixture of polycyclic aromatic hydrocarbons (PAHs) for modelling the aromatic infrared emission bands (AIBs) is shown. The laboratory spectra of these complex organic compounds represent a unique data set of high value for the astronomical community, e.g. they may be compared with the Herschel Space Observatory spectra (∼51–220 μm) of several astrophysical environments such as (proto-) planetary nebulae, H ii regions, reflection nebulae, star-forming galaxies and young stellar objects.