Cosmic Molecules and Clusters : Knockout Driven Reactions

Abstract: Fullerenes and PAHs (polycyclic aromatic hydrocarbons) are two families of carbon based molecules. These are both present in the interstellar medium, and are there believed to play important roles in various processes, including the formation of stars in the case of PAHs. This thesis presents studies on the structures and dynamics of fullerenes and PAHs and their weakly bound clusters, that all have relevance in an astrophysical context. Here, the focus is on knockout driven reactions in which a single atom is knocked out of a molecule or a molecular cluster as a result of Rutherford-like scattering processes. These are modelled by means of classical molecular dynamics simulations.The first study investigates knockout processes where a C60 molecule is collided with helium atoms at 166 eV in the centre-of-mass-frame, similar to the velocities in interstellar shocks. Using a combination of experimental measurements and molecular dynamics simulations we find that highly reactive C59 fragments can be created sufficiently cold to stabilise and survive indefinitely inisolation.Following the first study, we model the structures and stabilities of mixed clusters of C60 and C24H12 (coronene) molecules. We find that the two molecular species do not mix very well, but that they like to be in compact formations. For larger pure coronene clusters, we find that the most stable clusters contain two interacting stacks, forming a shape that looks similar to a “handshake”. These results are consistent with earlier modelling studies. Here, we show that such stacks also show up as subclusters in large mixed clusters.Finally, we use the most stable clusters from the second study as targets in collisions with 3 keV argon atoms. We find that the simulated mass spectra strongly resemble the corresponding experimental ones. These show that many various forms of new molecular structures, both fragments and large new molecules, are being formed, as a result of the collisions. Here, the simulations give information on the reaction pathways and on the structures of these new species. There are also examples of hydrogenated, but otherwise intact, fullerene and coronene molecules being formed.The mechanisms we have studied mimic inter- and circumstellar conditions where shockwaves and stellar winds drive particles (atoms and ions) at velocities similar to those studied here. The reactions covered in this work are thus likely to take place in such environments when carbon-based molecules and grains are energetically processed.