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

Periodic Reporting for period 1 - EUROPAH (The Extensive and Ubiquitous Role of Polycyclic Aromatic Hydrocarbons (PAHs) in Space)

Reporting period: 2016-10-01 to 2018-09-30

Interstellar polycyclic aromatic hydrocarbons (PAHs) are ubiquitous and lock up close to 15% of the elemental carbon in space. On Earth, PAHs are predominantly known as carcinogenic pollutants but understanding PAHs and their role in the Universe is a key question in astrophysics.

In astrochemistry PAHs: i) Act as probes of astrophysical and astrochemical conditions both in the Milky Way and in other galaxies, even at high redshift . ii) They act as catalysts for H2 formation as well as for increasing molecular complexity and possibly even the chemical evolution of life. iii) They interact with interstellar nano‐scale dust grains and control their surface properties. iv) They are found in comets and meteorites along with other organic molecules including amino acids ‐ thus forging a strong link between interstellar and solar system chemistry.

EUROPAH ETN brings together the leading astrophysics and astrochemistry research groups in Europe working observationally, theoretically and experimentally on PAHs in the ISM. These are supported by physics, chemistry, environmental science and nanoscience research groups specialising in theoretical and experimental surface science, plasma physics, computational chemistry, photo‐chemistry and molecular spectroscopy, as well as by three industrial participants that specialise in PAH destruction, scientific equipment for PAH studies and scientific outreach.

The research objectives of the EUROPAH ETN are to determine:
i) The spectral signature of PAHs and related species from the far‐IR through to far‐UV and how they can be used to trace interstellar chemistry and conditions (WP1).
ii) How PAHs and related species interact with energetic photons, ions and electrons in the gas phase (WP2).
iii) The role of PAHs and related species in the chemical evolution of solar system bodies such as comets and asteroids (WP3).
iv) The role played by PAHs and related species in the astrochemistry of the interstellar medium (WP4).
v) The physics and chemistry of PAH clustering and PAH agglomeration on nano‐scale grain surfaces, thus uncovering the relationship between PAHs, and carbonaceous and silcate grains, and the potential impacts in terrestrial environmental science (WP5).
The programme of research and training activities is progressing well and; a full cohort of ESRs is in place. There was a recruitment delay of roughly one year reflecting an increasingly demanding global recruitment environment. This delay has had no impact on the training of our cohort of ESRs as necessary steps were taken to maximise the training impact.
The ESRs have gathered 3 times for training activities: Aarhus, DK, Sept 2017 - Introductory training, training in public relations activities and a workshop on advanced surface science methods: Les Houches, FR. April 2018 - for a Summer school with introductions to astronomical observations & theory, experimental & quantum chemical techniques relevant for the study of PAHs in the interstellar medium of galaxies, and observing opportunities from the ground and with the to‐be‐launched James Webb Space telescope. This school incorporated a workshop on computational techniques: Edinburgh, UK, Oct 2018 - a scientific review of project progress led by ESR presentations.
Research progress is rapidly advancing and key instrumentation build is complete. For example, at RU, an atmospheric pressure chemical ionisation ion source has installed on their mass spectrometer to permit measurements of the IR spectra for ionized PAHs larger than coronene; and ULIV have developed a scalable gliding arc discharge plasma system for PAH processing. At UL, the OASIS instrument went through a major revision and is now fully operational.
UPS has recorded electronic spectra of some PAH cations in the VUV on the Soleil Synchrotron, that are being compared with computational work being undertaken at INAF; AU has successfully measured cross‐sections for H atom addition to coronene and pentacene and are in the process of preparing a publication. This effort is supported by the ESR at UL through quantum chemical study on the interaction of H atoms with pentacene in close collaboration with the node at the University of Milan; and HWU has fascinating, but tentative, IR results that might point to edge‐on interactions between ice and per‐hydroPAHs.
The collaborations established through our secondment plans have seen positive impact. Illustrative of this is the collaboration between the nodes at UCO and UPS which has produced a detailed IR study of the fragmentation of the 1‐methylpyrene cation that relatively recently appeared in the journal Chemical Physics Letters (2018, Vol. 698, pp. 206‐210); and the collaboration between UCO and RU through which the first vibrational spectra of the naphthalene dication have been recorded.
More importantly, we are starting to realise the added value of the network where cross-fertilization between the teams has led to new projects which were not originally included in the program. The Aarhus/Leiden/Milan study on hydrogenation of pentacene is a case in point.
We will produce vibrational and electronic spectra for specific PAHs, and use density functional theory calculations to understand and interpret these spectra. Vibrational spectra have already been acquired for several species and work is underway to construct computational models to assist with interpreting these results.
Identifying the dynamics by which PAHs break apart when subjected to photo excitations or plasma processing is a key objective. Fragmentation pathways following pumping with photons in the IR and VUV region can be expected and these measurements will be matched with time-dependent density functional theory models. These processes are known to occur in the ISM, but also in plasma processing of PAHs; a common technology used to clean pollutants in terrestrial environments. We are constructing a model to understand of the dynamics of PAH destruction in plasmas; testing the model with a simple CO2 molecule is complete and we will soon move to larger PAH molecules.
The interactions between PAHs and nano-sized grains are common in both the ISM and terrestrial environments. In both cases, understanding the interactions between these complex systems and water, in the presence of UV processing, is an expected outcome from EUROPAH. The equipment and ESRs to run these experiments are now in place so research progress is expected soon. Biological experiments aimed at understanding the impact of functionalised PAHs on terrestrial lifeforms in aquatic environments are also underway.
The catalytic effects of PAH molecules in the ISM, and the role that these molecules play in the dynamics of small molecule formation is another expected outcome. We have already exposed PAHs to sources of atoms, such as H and O, to produce functionalised PAHs and soon the role of these functionalised species in producing H2, OH, H2O and small carbon containing molecules will be investigated.
Finally, the link between individual PAHs and larger, carbonaceous grains will be examined. An experimental and theoretical approach will be used to understand the interactions and binding energies between different types of PAHs molecules and analogue dust grain surfaces. The ESRs are in place with training progressing well.