Using DESY's free-electron laser FLASH, scientists have recreated some of the harsh environment of interstellar space in the lab and analysed the reaction of astrochemical molecules to these conditions. The results show a comprehensive picture of the dynamics of polycyclic aromatic hydrocarbons (PAH) under extreme ultraviolet radiation in a vacuum – resembling the cosmic environment between the stars of our galaxy, the Milky Way. As the international team led by DESY scientists Bastian Manschwetus and Melanie Schnell write, the results promote understanding of organic chemistry in space. Their study has been published in the journal Nature Communications, and is featured in DESY's new Photon Science 2021 highlights report.
Organic chemistry is the reactions, compositions and properties of molecules containing carbon (C). It is particularly important for the chemistry of life. PAHs are an important group of organic compounds, consisting of carbon and hydrogen (H). “Polycyclic aromatic hydrocarbons are found in almost every corner of the universe, accounting for up to 20 per cent of all carbon in space,” explains Jason Lee from DESY, one of the paper's main authors. “These molecules play an important role in interstellar chemistry, providing reaction surfaces, aggregating into larger species such as fullerenes and fragmenting into building blocks for other organic molecules, among other things. Our work aims at understanding the reaction dynamics of PAHs following interaction with the ionising radiation found in interstellar space.”
The scientists investigated the response of the three small PAHs fluorene (C13H10), phenanthrene (C14H10), and pyrene (C16H10) to the extreme ultraviolet (XUV) radiation from DESY's free-electron laser FLASH. The XUV flashes were tuned to a wavelength of 30.3 nanometres, matching an important emission line of helium in interstellar space. For comparison: visible light has wavelengths between 400 and 800 nanometres.
The extreme ultraviolet photons can knock up to three electrons out of a PAH molecule, leading to a highly ionised state. With a specialised instrument, the CAMP endstation, and a super-fast camera, PImMS, the team could disentangle the complex fragmentation and ionization dynamics of the molecules. The analysis shows that all investigated PAHs respond extremely quickly following the absorption of the high energy radiation, redistributing the energy into atomic movement in much less than a picosecond (trillionth of a second). Scientists summarise these processes under the term relaxation. State-of-the-art theoretical calculation predict relaxation on the same timescale.
According to the data, doubly-ionised PAH molecules – so-called dications, where two electrons were kicked out of the molecule by the XUV photon – have a strong preference to split into two fragments each carrying a single electric charge (hence called monocations). The dications also showed a preference to fragment into two monocations accompanied by the neutral loss of two carbon atoms (C2). This is especially intriguing, as it mirrors the proposed mechanism for creating PAHs in the first place, where acetylene molecules are added together sequentially. Acetylene is a simple molecule with the formula C2H2. The experiments also recorded fragmentation from the triply-charged PAH molecules which will be reported in a follow up paper.
“Our results show that ultrafast relaxation may be ubiquitous amongst polycyclic aromatic hydrocarbons,” says Lee. Further experiments performed at FLASH in 2021 with a new set of PAHs are to corroborate this observation. These experiments provide valuable insights into the interaction of these abundant PAH molecules with interstellar radiation, revealing the products that would be formed in space. These ions and fragments form the building blocks for further molecules, shaping the organic chemistry of the cosmos.
Scientists from the universities of Oxford, Kiel, Lund, Gothenburg, Hamburg, Amsterdam, Göttingen, from the Radboud University in Nijmegen, Saint Petersburg State University, Kansas State University, Vrije Universiteit Amsterdam, European XFEL and DESY contributed to this research.
Publication
Time-Resolved Relaxation and Fragmentation of Polycyclic Aromatic Hydrocarbons Investigated in the Ultrafast XUV-IR Regime; J. W. L. Lee, D. S. Tikhonov, et al.; Nature Communications, 2021; DOI: 10.1038/s41467-021-26193-z
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