Photodissociation of aliphatic PAH derivatives under relevant astrophysical conditions

Abstract

The interaction of polycyclic aromatic hydrocarbons (PAHs) with vacuum ultraviolet (VUV) photons triggers the emission of the well-known aromatic infrared bands (AIBs) but other mechanisms such as fragmentation can be involved in this interaction. Fragmentation leads to selection effects that favor specific sizes and structures. We investigate the impact of aliphatic bonds on the VUV photo-stability of PAH cations under conditions applicable for photodissociation regions (PDRs). Cations of pyrene (C16H10) and coronene (C24H12) derivatives containing aliphatic bonds (methyl or ethyl sidegroups, superhydrogenation) are submitted to VUV (10.5 eV) photons over long timescales (~1000 s) in the cryogenic PIRENEA setup. The fragmentation cascades are analyzed with a simple kinetics model; fragmentation pathways, rates and branching ratios are derived. Aliphatic PAH derivatives are found to have a higher fragmentation rate and carbon loss compared to regular PAHs. The fragmentation of PAHs with alkylated sidegroups forms species with peripheral pentagonal cycles, which can be more stable than the bare PAH cations. This stability is quantified and the most stable species, for which there is an effective competition of the fragmentation with isomerization and radiative cooling are identified. This work supports a scenario in which the evaporation of nanograins with a mixed aliphatic and aromatic composition followed by VUV photoprocessing results in both the production of the carriers of the 3.4um AIB by methyl sidegroups and in an abundant source of small hydrocarbons at the border of PDRs. An additional side effect is the efficient formation of stable PAHs that contain some peripheral pentagonal rings. Our experiments also support the role of isomerization processes in PAH photofragmentation, including the H-migration process, which could lead to an additional contribution to the 3.4um AIB.

Astronomy and Astrophysics, in press

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