Differing Isomeric Responses used as a Barometer for Organic Materials in the Cosmos
Authors: Wren Montgomery, Jonathan S. Watson, Mark A. Sephton
Affiliation: Impacts and Astromaterials Research Centre, Department of Earth Science and Engineering, Imperial College, London, UK
We showed that a family of organic molecules having the same atomic composition but different structures have substantially different responses to the application of high pressure. This is the first potential cosmic barometer capable of recording high pressure events throughout the history of the universe.
Organic matter is common throughout the universe. It is found in interstellar and circumstellar clouds, asteroids, meteorites, comets, planets and satellites [1,2]. Taking a cue from terrestrial geochemistry, where isomer distributions of aromatic hydrocarbons can be used to determine formation environments (primarily temperature), we were intrigued by the possibility of a similar indicator for astronomical environments.
In order to reach high pressures in the laboratory, we used the diamond anvil cell (Figure 1). In this device, two modified brilliant-cut gem quality diamonds are mounted culet-to-culet and gently pressed against each other. The broad transparent window (UV to Far Infrared) of the diamond anvils means a variety of techniques can be used for in-situ measurements.
Figure 1. (a) Schematic of high-pressure synchrotron-source FTIR spectroscopy measurements parallel to the synchrotron beam. Not to scale. (b) Schematic of high-pressure synchrotron-source FTIR spectroscopy measurements in the plane of the sample. Not to scale. (c) Microphotograph of 1,7-dimethylnaphthalene loaded into the diamond anvil cell. The ruby is the sphere toward the center of the sample chamber ( © 2014. The American Astronomical Society).
In our recent work , we used synchrotron source FTIR spectroscopy in the diamond anvil cell (Figure 1) at the Swiss Light Source and SOLEIL Synchrotron (France) to collect FTIR spectra for 5 isomers of dimethylnaphthalene (DMN). DMNs were chosen because this type of organic structure has been suggested as the cause of some unidentified infrared bands  and is present in meteorites .
By plotting peak center against pressure, we determined the relative effects of pressure on the isomers (Figure 2). Overall, the greatest changes occurred in 1,8-DMN, possibly due to the greater void space in the crystal at ambient conditions. Meanwhile, 1,5-DMN shows very little change with the application of high pressure, contrary to the previously known instability at ambient conditions .
Figure 2. Compression and decompression peak fit data for 1,8-, 1,5-, 1,7-, 2,6-, and 2,7-dimethylnaphthalene. Each data point represents the center of a peak associated with that material at a single pressure ( © 2014. The American Astronomical Society).
Overall, the responses of dimethylnaphthalenes to pressure conform to those observed in response to heat. The response of 1,5-DMN to pressure is contrary to those changes brought about by high temperature and reveals that although the molecular consequences of heating and pressurizing are often the same, the two mechanisms are very distinct and can generate opposing effects.
This suggests that measurement of infrared spectra of organic samples from the cosmos, either on samples in the laboratory or using remote instruments on Rovers or Landers can be used to quantify the thermodynamic history of the universe.
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