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Fragmentation is an important decay mechanism for polycyclic aromatichydrocarbons (PAHs) under harsh interstellar conditions and represents apossible formation pathway for small molecules such as H2, C2H2, C2H4. Our aimis to investigate the dissociation mechanism of superhydrogenated PAHs thatundergo energetic processing and the formation pathway of small hydrocarbons.We obtain, experimentally, the mass distribution of protonated tetrahydropyrene(C16H15 , py+5H+) and protonated hexahydropyrene (C16H17+, py+7H+) uponcollision induced dissociation (CID). The IR spectra of their main fragmentsare recorded by infrared multiple-photon dissociation (IRMPD). Extendedtight-binding (GFN2-xTB) based molecular dynamics simulations are performed inorder to provide the missing structure information in experiment and identifyfragmentation pathways. The pathways for fragmentation are further investigatedat a hybrid-density functional theory (DFT) and dispersion corrected level. Astrong signal for loss of 28 mass units of py+7H+ is observed both in the CIDexperiment and the MD simulation, while py+5H+ shows negligible signal for theproduct corresponding to a mass loss of 28. The 28 mass loss from py+7H+ isassigned to the loss of ethylene (C2H4) and a good fit between the calculatedand experimental IR spectrum of the resulting fragment species is obtained.Further DFT calculations show favorable kinetic pathways for loss of C2H4 fromhydrogenated PAH configurations involving three consecutive CH2 molecularentities. This joint experimental and theoretical investigation proposes achemical pathway of ethylene formation from fragmentation of superhydrogenatedPAHs. This pathway is sensitive to hydrogenated edges (e.g. the degree ofhydrogenation and the hydrogenated positions). The inclusion of this pathway inastrochemical models may improve the estimated abundance of ethylene.

Top-down formation of ethylene from fragmentation of superhydrogenated polycyclic aromatic hydrocarbons