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A laboratory study of astronomically relevant large polycyclic aromatic hydrocarbons (GRAND-PAHs)

Periodic Reporting for period 1 - GRAND-PAHs (A laboratory study of astronomically relevant large polycyclic aromatic hydrocarbons (GRAND-PAHs))

Reporting period: 2019-09-01 to 2021-08-31

Astronomical observations reveal that large polycyclic aromatic hydrocarbons (PAHs) exist in copious amounts in the interstellar medium (ISM). They contain up to 20% of the entire carbon budget in the Universe and as such need to be understood in terms of their origins (how they form) and their eventual fate (how they are destroyed). The overall objective of this project is to measure the fragmentation pathways of very large polycyclic aromatic hydrocarbons (PAHs) in a laboratory setting by utilizing several complementary radiation techniques to gain a deeper understanding of the nature of GRAND PAHs and their overall role in the chemistry and physics of our Universe.
The major finding of this fellowship is that (GRAND)PAHs fragment in universal ways, i.e. no matter the size or shape of PAHs, their fragmentation products appear to be almost identical. This gives rise to particular carbon clusters that may easily build up in the universe before being inducted into molecular clouds. More importantly, these fragments are likely candidates as carriers of the Diffuse Interstellar Bands (DIBs), which are a century old mystery at the time of writing.
In Leiden, the crux of the fellowship’s work was done on the “instrument for Photodynamics of PAHs” (i-POP). When I started the fellowship, i-POP was undergoing some malfunctions. During that time, I worked on a project I had initiated with my supervisor prior to the fellowship concerning the photoionization of the Buckminsterfullerene C60 which has been found in several regions in space and is known to form from the irradiation of PAHs. This project became an integral part of the fellowship leading to a publication in PCCP where it was chosen for the back cover of the issue and led to my first invited talks. A follow up project is currently underway in collaboration with theoretical chemists to shed further light on the complications wrought by the C60 molecule losing an electron.
As restrictions due to the pandemic eased, I both finished repairing i-POP and I completed a series of measurements that ultimately showed that PAH molecules of different symmetries and different sizes, ostensibly fragment in near identical ways upon continued laser irradiation. The work showed that certain “magic number” carbon clusters (i.e. arrangements of a magic number of carbon atoms that arrange themselves to form a very stable molecular structure) are preferentially formed when PAH are exposed to high levels of radiation (such as in space). These results are very exciting as these fragmentation processes are designed to mimic what happens to PAHs in highly irradiated interstellar regions. More experiments are envisioned and will be performed under my supervision in the additional year I will spend in Leiden.
Interestingly, these carbon clusters are predicted to be in the shape of rings and are part of a very interesting group of molecules that are becoming a hot topic in modern science. These experiments show that these carbon rings are ideal candidates for a new group of molecules that could be found in space and further to the point, could be carriers of the Diffuse Interstellar Bands (DIBs), a century old spectroscopic mystery.
To investigate the interactions between PAHs and ultraviolet (UV) radiation that is comparable to that found in the interstellar radiation fields, beamtime applications to the SOLEIL synchrotron and the Swiss Light Source (SLS) were written and accepted. At SOLEIL, the intent was to investigate how large (GRAND) PAHs become multiply ionized (lose more than one electron) as they absorb UV photons. At SLS the intent was to investigate the ionization of smaller, nitrogen-containing PAHs, to learn how they interact differently with UV photons than pure PAHs comprising carbon and hydrogen atoms.
To obtain very large PAHs that are not commercially available, I connected with researchers from the Max Planck institute for Polymer research in Mainz, Germany, who were interested in collaborating on a series of projects were I could inspect the physical chemical characteristics of these large PAHs they specialize in synthesizing.
As the COVID-19 pandemic hit approximately six months after the project started, this caused some significant delays in the projects, resulting in the granted SOLEIL beamtime being delayed until the fellowship officially ended. To combat the effects of the pandemic, Leiden University offered me a contract extension for one year during which I could finalize all projects pertaining to the fellowship that were delayed by the pandemic. In turn, I took on the responsibility as assistant lecturer in a course dedicated to statistics and data analysis for astronomy students.
I went to free electron laser for infrared experiments (FELIX) laboratory in Nijmegen for a secondment where I recorded infrared spectra of (GRAND) PAH molecules. The results of this secondments were interesting and are currently being analyzed but at FELIX I received training relevant to free electron lasers and using new experimental techniques that will benefit my career in the long run.
Preliminary preparation for my secondment at FELIX have been done. The measurements were done on chlorinated, alkylated and some pure GRANDPAH species. The objective was to find a direct link between the pure species and the chlorinated / alkylated ones such that molecules that are more soluble in organic solvents can be used as a proxy.
To disseminate my work, I attended a number of conferences, both virtually and in person, and I gave my first two invited talks of my career. Furthermore, I gave a popular talk on astrochemistry in my hometown Reykjavík as part of the “Nerdnite Reykjavík” series which is dedicated to interesting talks on a wide variety of subjects.
I enrolled in several courses offered by Leiden university that will help develop my management skills. I am now in a course focused on training PhDs after finishing a course on feedback in an international context. Upcoming are further courses that include writing grant proposals, networking skills and basic management and leadership skills.
There are multiple publications in preparation from the work performed during this action and will be published in the additional year I will spend in Leiden. There are two publications in preparation on my work on i-POP that will be submitted in the coming two months. And there will be at least three more papers from my work at FELIX, and the SOLEIL beamtime that had to be delayed until 2021.
There are many works currently underway. Three publications on results from i-POP, a follow up publication to my paper on the photoionization of C60. From the FELIX secondment there are two papers in preparation, one on perchlorinated PAHs and one on alkylated PAHs and likewise from the SOLEIL beamtime on GRAND PAHs, there are two publications in preparation.
Societal impacts are difficult to quantify, but the crux of the project is highly relevant to one of the longest standing scientific mysteries of our time, namely the Diffuse Interstellar Bands. The aforementioned carbon clusters that form upon the destruction of PAH molecules are ideally suited candidates as carriers of some of the DIBs but detailed work on the spectroscopy of these carbon clusters is needed and is something I would like to work on as a future career trajectory.
GRAND PAHs intended for future experiments