  Fragmentation is an important decay mechanism for polycyclic aromatic
hydrocarbons (PAHs) under harsh interstellar conditions and represents a
possible formation pathway for small molecules such as H2, C2H2, C2H4. Our aim
is to investigate the dissociation mechanism of superhydrogenated PAHs that
undergo 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+) upon
collision induced dissociation (CID). The IR spectra of their main fragments
are recorded by infrared multiple-photon dissociation (IRMPD). Extended
tight-binding (GFN2-xTB) based molecular dynamics simulations are performed in
order to provide the missing structure information in experiment and identify
fragmentation pathways. The pathways for fragmentation are further investigated
at a hybrid-density functional theory (DFT) and dispersion corrected level. A
strong signal for loss of 28 mass units of py+7H+ is observed both in the CID
experiment and the MD simulation, while py+5H+ shows negligible signal for the
product corresponding to a mass loss of 28. The 28 mass loss from py+7H+ is
assigned to the loss of ethylene (C2H4) and a good fit between the calculated
and experimental IR spectrum of the resulting fragment species is obtained.
Further DFT calculations show favorable kinetic pathways for loss of C2H4 from
hydrogenated PAH configurations involving three consecutive CH2 molecular
entities. This joint experimental and theoretical investigation proposes a
chemical pathway of ethylene formation from fragmentation of superhydrogenated
PAHs. This pathway is sensitive to hydrogenated edges (e.g. the degree of
hydrogenation and the hydrogenated positions). The inclusion of this pathway in
astrochemical models may improve the estimated abundance of ethylene.
