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Unraveling interstellar chemistry with broadband microwave spectroscopy and next-generation telescope arrays

Periodic Reporting for period 4 - ASTROROT (Unraveling interstellar chemistry with broadband microwave spectroscopy and next-generation telescope arrays)

Reporting period: 2019-05-01 to 2020-04-30

The goal of the research program, ASTROROT, is to significantly advance the knowledge of astrochemistry by exploring its molecular complexity and by discovering new molecular classes and key chemical processes in space. Besides a detailed chemical inventory of interstellar space (more than 200 molecules have been identified to date), it is equally important to elucidate the reaction mechanisms under which these molecules are formed and can react further. So far, mostly physical reasons were investigated for the observed variations in molecular abundances. Within ASTROROT, I studied the influence of chemistry on the molecular composition of the Universe by combining unprecedentedly high-quality laboratory spectroscopy and pioneering telescope observations. We designed and employed newly developed, unique broadband microwave spectrometers with the cold conditions of a molecular jet and the higher temperatures of a static sample cell to mimic different interstellar conditions. Their key advantages are accurate transition intensities, large reduction in measurement times, and unique mixture compatibility. We also coupled it with an electric-discharge source for the generation of exotic molecules potentially relevant for astrochemistry.
Our laboratory experiments motivate and guide astronomical observations and enable their interpretation – all this happening in a strong network of fruitful collaborations. Our studies had a particular focus on organic molecules in astrochemistry that can relate to the long-standing quest to understanding the origin of life. There are hypotheses that biomolecules from space initiated or fostered the development of life on Earth. For example, we investigated potential precursors of the building blocks of life, such as the amino alcohols alaninol, valinol, leucinol, and isoleucinol to unravel their role in astrochemistry.
Different scenarios for reaction mechanisms under which these molecules are formed and can react further are discussed, among them the interplay of top-down vs. bottom-up processes, which we investigated using cutting-edge molecular spectrometers. We used large areas of the electromagnetic spectrum to probe different processes, where we mainly focused on polycyclic aromatic hydrocarbons (PAHs), an important molecule class in space. As such, we extended our activities to also explore the interaction of astrochemically relevant molecules with harsh radiation up to the XUV and X-ray frequency range provided by free-electron lasers and synchrotron light sources. We observed a striking interplay between ionization and fragmentation of PAHs upon XUV excitation, with the loss of acetylene units being one of the preferred ones.
The ASTROROT research program was very successful, with twelve studies being published in international, peer-reviewed scientific journals so far; several further publications are still in preparation. Furthermore, it was central to establish high-level and fruitful collaborations in the astrochemistry field.
The results of ASTROROT do significantly foster our understanding of astrochemistry. The research goes far beyond the state-of-the-art: We developed and used cutting-edge techniques in the laboratory. These techniques now enable studies for this important frontier of physics and chemistry that previously would have been prohibitively time-consuming or even impossible. Our studies significantly extended the spectroscopic knowledge of PAHs and of small biomolecules in vibrationally excited states – important tracers in space also with interest for the origin of life. Furthermore, we gained information about new formation pathways of complex molecules at the molecular level by exploring the extreme conditions of plasma sources.
In the following, some highlights are summarized. To spectroscopically characterize the molecules of interest over a broad frequency range, we developed a new broadband rotational spectrometer covering the 18-26 GHz frequency range. This frequency range overlaps with a number of radio telescopes in the world, so that direct laboratory measurements from this range are particularly relevant for the community. This spectrometer is based on the new segmented approach, which is a low-cost approach and thus particularly appealing for smaller groups as well as teaching laboratories, and as such an important dissemination aspect of ASTROROT.
Our studies had a particular focus on organic molecules in astrochemistry that can relate to the long-standing quest to understanding the origin of life. The strength of our approach is that we can cover a large frequency range and thus generate very accurate and precise line lists for radioastronomic searches. There are hypotheses that biomolecules formed in space initiated or fostered the development of life on Earth. Our studies contributed to unraveling the chemical inventory of interstellar space also with respect to potential precursors of the building blocks of life, such as the amino alcohols alaninol, valinol, leucinol, and isoleucinol or imidazole, which is a heteroatomic ring. We provided a detailed spectroscopic characterization of these molecules to search for them in interstellar space, including their rare isotopologues.
In the course of the ASTROROT program, we developed a new research line focusing on the structure and photophysics of polycyclic aromatic hydrocarbons (PAHs). Besides detailed spectroscopic characterizations of PAHs monomers, we also investigated PAH-water complexes, which can provide important insights into the first steps of ice grain formation. These results gained significant attention also beyond the astrochemistry community, because such ice grains are understood to be at the heart of a rich chemistry in space.
Using different light sources, we also investigated the photophysics of PAHs, where we found interesting ultrafast break-up processes of these molecules upon harsh radiation. Using the plasma environment of an electric discharge source, we studied the interplay between bottom-up vs. top-down processes. The results are supporting the high chemical flexibility in space and the importance of bottom-up and top-down processes.
The research program includes advanced spectrometer development and their application to astrochemical problems. The results are communicated in scientific and public talks as well as in peer-reviewed publications in international scientific journals.
The principles of our advanced spectrometer design will certainly have impact for other groups, also outside the astrochemistry community, as well as teaching laboratories of Universities, because of its low-cost design without sacrificing the performance.
We built up a dedicated research line on PAHs, where we cover a large area of the electromagnetic spectrum in our experiments – from the microwave to the X-ray range. With this broad approach we could address many different aspects of PAHs; from structures of monomers and water clusters to reaction mechanisms and ultrafast photophysics. Our studies on (chiral) biomolecules related to the question on the origin of life. The results were communicated at several conferences and workshops and already triggered related work.
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