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The Extensive and Ubiquitous Role of Polycyclic Aromatic Hydrocarbons (PAHs) in Space

Periodic Reporting for period 2 - EUROPAH (The Extensive and Ubiquitous Role of Polycyclic Aromatic Hydrocarbons (PAHs) in Space)

Reporting period: 2018-10-01 to 2021-03-31

Molecules play a key role in controlling the physics and chemistry of the Interstellar Medium – the space between the stars. Molecules impact on the distribution of energy in the Interstellar Medium and this in turn controls where and when stars and planetary systems form. One class of molecules, the polycyclic aromatic hydrocarbons or “PAHs”, play a particularly important role. Close to 15 % of all of the carbon in interstellar space exists in a PAH molecule. PAHs play key roles in the ionization balance and heating of the interstellar gas, and thus influence the phase structure of the interstellar medium. Everywhere you point a telescope in space, you will observe the presence and influence of PAHs. PAHs are also central to the chemical complexity of space and the organic inventory of regions of star and planet formation. The PAH molecules present when planetary systems are forming may influence the chemical species that evolve on that planet. Understanding PAHs and their role in the Universe is thus a key question in astrophysics.
EUROPAH brought together, astronomers, astrophysicists, astrochemists, biologists, social scientists and plasma physicists to create an interdisciplinary research and innovation environment. Our aim was to use cutting edge experimental and computational approaches to gather fundamental research data in laboratory settings, to understand how PAH molecules behave in the Interstellar Medium. The data from the EUROPAH network will help astronomers to interpret the data from their observations, and offers a firm understanding to the observations made using past, present and future telescopes. On Earth, PAHs are predominantly known as carcinogenic pollutants, as found, for example, in motor vehicle exhaust fumes and petroleum‐related pollution. Some of the methods used to understand the chemistry of PAHs in the Interstellar Medium may also help us understand the behaviour of PAHs on our own planet.
A new generation of 18, diverse researchers were recruited and trained to be capable of interacting and working together in a multidisciplinary world. They demonstrated many new aspects in the physics and chemistry of PAH molecules. Infrared spectra of PAH species is the core means of identifying these molecules via observational astronomy. EUROPAH produced IR spectra of a vast array of PAH molecules, in various chemical states and developed the computational methods necessary for understanding these spectra and predicting how other PAH species might be identified in the ISM. The interaction of PAHs with ionising radiation demonstrated how these molecules can recycle energy in the Interstellar Medium to generate new chemical species. PAHs interact with interstellar nano‐scale dust grains both on Earth and in the ISM, and EUROPAH researchers demonstrated how PAHs interact with other chemical species, such as ices, dust grain surfaces, nanoparticle, and atomic H and O to generate larger but also smaller chemical entities; identifying some species that might act as precursors for the chemistry building blocks we think of as essential for life on Earth. Finally, we learned how incorporation of interactions with the public as a compulsory part of our researchers’ learning experience provided a step towards a different culture of research.
- The spectroscopy work package produced both vibrational and electronic spectra for specific PAHs, and used density functional theory calculations to understand and interpret these spectra. The cryogenic ion trap instrument - FELion - at the FELIX Laboratory has developed into a versatile tool to perform high-resolution IR studies on a large variety of PAH ions.
- The energetic processing work package identified the dynamics by which PAHs break apart when subjected to photo excitations, pumping with photons in the IR and VUV region, or plasma processing. Machine learning methods have also been developed to better predict the outcomes for future experiments.
- Results form the solid-state work package help us to understand the interactions between PAHs and nano-sized grains, common in both the ISM and terrestrial environments.The results compare UV processing of PAHs in gaseous and aqueous environments, and elucidate new chemical pathways to potentially prebiotic molecules, i.e. chemical pathways in the ISM for the production of the molecules essential for life. From a terrestrial perspective, biological experiments determined the concerted effects, induced by PAHs and TiO2 nanoparticles, on gene expression in embryonic zebrafish.
- The astrochemistry work package determined the catalytic effects of PAH molecules in the ISM, and the role that these molecules play in the dynamics of small molecule formation. Results demonstrate the effects of edge-structure, functionalisation and curvature on the chemistry of PAHs and PAH reactions with atomic species.
- The carbonaceous grains work package studied the clustering of small and fragmented PAH molecules, and the ability of these species to act as seeds for the growth of larger carbonaceous materials.
- Research on outreach investigated the experience of EUROPAH regarding the introduction of public engagement as a compulsory part of the ESR’s learning experience and as a research topic, in a step towards a different culture of research.
- Our results were communicated to the field of of observational astronomy through traditional academic channels and via population of the relevant databases and distribution of publically available datasets. The early-stage researchers in the network carried the majority of these dissemination activities through publication and deliverance of scientific presentations; this in turn, enhanced the writing and oral skills of those involved. To reach the public, our researchers used two-pronged approach – with one track based online, and a second track based on face-to-face interaction, culminating in the EUROPAH pop-up shop.
We have collected IR spectra, for a vast array of PAH molecules in various states of ionisation and chemical functionalisation, using high-resolution techniques at the FELIX light source. This led to the development of new computational and machine learning methods that can help to understand molecular structure with unprecedented efficiency. Several of these datasets have already led to the proposed identification of species in the ISM. We have quantitatively resolved the fragmentation pathways for the series of PAH species upon excitation using either VUV or plasma-environments, for both gas phase and solid-state samples. Computational methods, including machine-learning approaches, have been developed to support these experimental observations and a patent has been submitted relating to plasma-based decomposition of PAH species in terrestrial environments.
We have elucidated the influence of edge-effects, chemical functionalisation and curvature on the reaction dynamics between PAH molecules and reactive radical-species. This will help to build a model for PAH-related chemistry that can take place in the ISM.
Biological experiments determined the concerted effects, induced by PAHs and TiO2 nanoparticles, on gene expression in embryonic zebrafish. These results demonstrate the breadth of research conducted in EUROPAH – from measurements to understanding the processes of the Interstellar Medium, to the processes working in our natural environments.
The EUROPAH consortium