ASTROCHEMISTRY · The role of pentagons in the conversion of Polycyclic Aromatic Hydrocarbons into fullerenes in the interstellar space

The role of pentagons in the conversion of Polycyclic Aromatic Hydrocarbons into fullerenes in the interstellar space

The project has achieved most of its objectives and milestones for the period, with relatively major improvements and additional experiments (including additional beamtime) which was achieved efficaciously within the same timeline and resources. Scientific objectives: As mentioned previously, the principle objective of this project was understanding the organic inventory of space and the processes that regulate their abundances by dissociation of large organic molecules. The crux of the work carried out entailed the photo-fragmentation of PAHs with the instrument for Photodynamics of PAHs (i-PoP) in the Laboratory for Astrophysics (LfA) in Leiden. The measurements performed on i-PoP at the LfA have revealed that PAHs appear to follow a universal fragmentation mechanism that gives rise to identical products, namely ionized carbon clusters in the form of cyclo[n]carbons. These end products could be the final step of the top-down breakdown processes in the photodissociation regions of the interstellar medium. It is found that, close to stars, the abundance of C60 increases rapidly while the abundance of PAHs decreases. This has been attributed to photochemical fragmentation and isomerization processes under the influence of the strong stellar ultraviolet (UV) radiation field but the details of these processes are not yet understood. A detailed study of the bright reflection nebula, NGC 7023, has given evidence that the evolution of the profile of the mid-IR bands is related to the chemical evolution under the effect of UV photons. Neutral and cationic C60 – identified through their unique IR and visible spectrum – are the largest molecular species identified in space. The presence of pentagons in a planar graphene-flake leads to bending of the molecular structure enabling the closure upon itself and formation of a 3D structure. , Pentagon formation is thus a key step in the photochemical transformation of PAHs into fullerenes. Spectroscopic evidence for pentagon formation upon PAH photo-dissociation has been reported in the laboratory for small PAH species, containing up to three aromatic rings. , These experiments suggest that PAHs with only hexagonal rings could be the parent molecules (Parent-PAHs) for the formation of PAHs with pentagonal ring(s) in adequate conditions. In addition, some PAHs with pentagons may be more photochemically stable than PAHs of similar size and therefore represent likely candidates for the interstellar molecular inventory. However, the photochemical evolution of PAHs with pentagons is largely unexplored and the generality of this conclusion is unknown. Given the presence of C60 in space, the likely astrochemical connection of C60 and PAHs in space, and the potential photochemical stability of pentagon containing PAHs, a focused study to improve the understanding of such species is done in my research.

Текст от CORDIS, на английски · Данни: CORDIS, © Европейски съюз