Missing cosmic deuterium localized
Nearly all deuterium in our universe was formed during the era after the Big Bang. Since then, it has mostly been depleted by stars, that turn deuterium and hydrogen into radiation. Therefore, in most parts of space, the ratio between hydrogen and deuterium is constant. However, there are parts of the interstellar medium, where stellar consumption alone cannot account for all of the deuterium depletion. There is a massive deficit. Using mass spectrometry and the infrared spectroscopy available at HFML-FELIX, researchers were able to show that polycyclic aromatic hydrocarbons (PAHs) are very likely to absorb this missing deuterium, at a much faster rate than previously thought.
PhD student Sandra Wiersma in the lab, using a solution of PAHs in methanol, to make a fine spray from which the ionic PAHs can be selected and studied in the mass spectrometer.
Infrared telescopes vs. spectroscopy
PAHs are known to contain much of the interstellar carbon, and to react with both hydrogen and deuterium. Through the use of infrared telescopes in orbit around the earth, astronomically observed signature lines can give rough estimations of the deuterium storage. However, this part of the astronomical infrared spectrum is difficult to observe through the currently used telescopes, and the actual chemistry is poorly understood due to a lack of experimental work.
HFMl-FELIX researchers present an experimental study of how exposure to light can lead to an increase of deuterium storage on interstellar PAHs. They exposed deuterium-containing PAHs to UV light, and showed that they have a much higher preference for the loss of hydrogen over deuterium. Using infrared spectroscopy they were able to see the structures of the molecules before this process started, and could conclude that there must be a shuffling or scrambling of the structure of the molecule to allow the molecule to lose so much more hydrogen than deuterium. The deuterium must end up locked in an aromatic position on the molecule from where the bond is too strong to break. Using computer simulations, they showed that the energy for such a scrambling process is much lower than the energy needed to break a hydrogen bond, and that it is a very fast reaction.
PAHs with scrambled, aromatic deuterium have very clear infrared signatures that should be detectable in space. Through the reevaluation of old astronomical data and the additional observations from the soon-to-be launched James Webb Space Telescope, it will become possible to see how much deuterium is actually stored in PAHs.
Photolysis-induced scrambling of PAHs as a mechanism for deuterium storage, Sandra D. Wiersma, Alessandra Candian, Joost M. Bakker, Jonathan Martens, Giel Berden, Jos Oomens, Wybren Jan Buma and Annemieke Petrignani, Astronomy & Astrophysics 635, A9 (2020)