Astronomical spectroscopy of the interstellar medium has allowed the identification of more than 160 molecular species. These measurements are highly reliant on laboratory-based spectral predictions.1 In the infrared, several of the spectral bands apparent in spectra of many parts of the Milky Way and elsewhere in interstellar space are yet to be identified, representing an interesting area of research.
Model systems that have been studied include polycyclic aromatic hydrocarbons (PAHs) which, following thermal treatment, present spectra similar to those observed astronomically; it has also been suggested that mixed PAHs and other aliphatic/olefinic compounds leads to spectral signals that are quite close to those of coal and petroleum fractions.2,3
Laboratory-based infrared spectroscopy is obviously a very important tool in investigating the infrared emissions seen in astronomical observations. By studying model systems here on earth, astrophysicists are able to identify mechanisms that should produce the compounds visible in the distant dust and gas clouds of our galaxy and those farther afield.
Do IR signals of the interstellar medium have terrestrial analogues?
One class of interstellar objects that is of interest is dust. Although solid matter makes up only a tiny fraction of the total, according to Jäger et al it is expected to have an important role in catalysing the production of many molecules now thought to be present in the interstellar medium.4 Their work focused on characterising cosmic dust analogues (CDAs) across a range of spectroscopic techniques, including infrared, with the goal of generating databases of dielectric functions that will aid the identification of astronomical dust species. They used the Specac HTHP cell for this work, as it allowed reflectance spectra of the CDAs to be recorded at temperatures up to 973 K (700 °C).
Vinogradoff et al have investigated the formation of carbonaceous chondrites, which fall to earth as meteorites. It is thought that the organic content of these minerals is altered during the process of accretion that forms the parent asteroids. To study this, the compound Hexamethylenetatramine (HMT) was subjected to asteroidal hydrothermal conditions at 150 °C for durations up to 31 days, after which the various compounds formed were characterised5. Infrared spectroscopy was used in the study to characterise the insoluble organic matter (IOM) produced under these conditions, along with elemental analyses and X-Ray Absorption Near Edge Spectroscopy (XANES). ATR-FTIR spectra of IOM samples from the experiment were recorded using a Specac Quest diamond ATR accessory. It showed that the IOM forms in experiments longer than 7 days but is relatively stable thereafter and shows the presence of nitrogen-rich imines and oxygen-rich ketones, phenols, carboxyls, and hydroxyls.
Coal-based models
The suggestion that unidentified infrared emissions might fit a petroleum and coal-based model has been reviewed by Cataldo et al.3 Their review (based mainly on their own earlier work) considered the model alongside the mixed aromatic aliphatic organic nanoparticle (MAON) model and the carbon arc model in which simple materials can be converted into more complex fullerenes and fulleranes. Infrared spectra of all the potential substrates were recorded at temperatures in the range of -180 °C to +50 °C (or +250 °C in the case of the coal) using the Specac Variable Temperature cell. They conclude that the MAON model covers the petroleum and coal models well and may be unified with it, since the thermal and radiation treatment (of the kind that might occur in shockwaves in space) of MAON-like molecules produces carbonaceous species with characteristics similar to coal and petroleum fractions.
References
1. NIST. Spectroscopic identification of interstellar molecules. https://www.nist.gov/pml/sensor-science/optical-radiation/spectroscopic-identification-interstellar-molecules (2014).
2. Yang, X., Glaser, R., Li, A. & Zhong, J. X. The Carriers of the ‘Unidentified’ Infrared Emission Features: Clues from Polycyclic Aromatic Hydrocarbons with Aliphatic Sidegroups. (2017) doi:10.1016/j.newar.2017.01.001.
3. Cataldo, F., Garcia-Hernandez, D. A. & Manchado, A. Petroleum, coal and other organics in space. (2020).
4. Jäger, C. et al. Recent Results of Solid-State Spectroscopy. (2012) doi:10.1017/S1743921311025166.
5. Vinogradoff, V., Bernard, S., Le Guillou, C. & Remusat, L. Evolution of interstellar organic compounds under asteroidal hydrothermal conditions. Icarus 305, 358–370 (2018).
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