Polycyclic Aromatic Hydrocarbons (PAHs) have recently been put forward as a possible starting point for the so-called "top-down" chemistry in the interstellar medium. Indeed, after the removal of the H atoms, PAHs can lose C atoms and lead to the formation of fullerenes and other intermediate species. In this experimental work we focus on the first step by studying the dehydrogenation pattern of large PAHs (<i>N</i><sub>C</sub> ≥ 24) following irradiation by a laser source. We follow the energy, wavelength, size and edge structure dependence and we also study the hydrogenation state of the first few fragments produced after the onset of carbon losses. The molecules studied lose hydrogen sequentially and fragments with even number of H are predominant. This pattern appears to be independent of the edge structure and/or size of the molecules. In terms of the wavelength dependence, the only difference observed is related to the cross-section of the parent specie, while the partially hydrogenated fragments again show no significant variation. Some differences are observed once the carbon loss is activated and they seem to depend only on the size of the molecules. PAHs with less than 32 C atoms lose carbon one by one; also the hydrogenation of the fragments with an odd number of carbons show predominantly an odd number of hydrogens. For molecules with more than 32 C atoms, the carbon losses proceed in steps of two and the hydrogenation of the fragments becomes progressively less significant. We will discuss the astrophysical implications of our findings.

Dehydrogenation of PAHs: First steps towards fullerenes in the ISM

249th National Meeting (2015)

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