Impact of aliphatic bonds on the stability of VUV photo-processed PAHs under relevant astrophysical conditions

International audienceStudying the interaction of polycyclic aromatic hydrocarbons (PAHs) with VUV light is crucial for the understanding of the physical and chemical evolution of photodissociation regions (PDRs). In particular, this interaction induces the well-known aromatic infrared bands (AIB) emission [1] but other mechanisms such as fragmentation [2] can be involved, leading to selection effects favoring specific sizes and structures. In this context, despite their lower stability, PAHs containing aliphatic C-H bonds are considered as good candidates for the 3.4 μm AIB [3,4]. We have recently combined a 10.5 eV photon source with our dedicated cryogenic ion trap PIRENEA [5] in order to investigate the impact of aliphatic bonds on the VUV photo-stability of PAH cations with conditions applicable for PDRs [6]. The coronene cation (C24H12+) and its alkylated derivatives (with methyl or ethyl sidegroups) were VUV photo-processed over long timescales (~1000 s). Their fragmentation cascades were analyzed with a simple kinetics model which enabled to derive fragmentation pathways, rates and branching ratios.Alkylated coronene derivatives are found to have a higher fragmentation rate and carbon loss compared to the bare coronene. However, their VUV fragmentation cascade also leads to the formation of species carrying a peripheral pentagonal cycle, which is as stable as coronene. The stability of each detected species is quantified and the most stable ones, for which there is an effective competition of fragmentation with isomerization and radiative cooling, are identified. This work supports a scenario in which the evaporation of nanograins with mixed aliphatic and aromatic composition followed by VUV photo-processing results in both the production of the carriers of the 3.4µm AIB by methyl sidegroups and in an abundant source of smaller hydrocarbons at the border of PDRs. This process also results in the formation of PAHs containing pentagonal ring. Finally, our study supports the role of isomerization processes in PAH photofragmentation, including the H-migration process [7], which could lead to an additional contribution to the 3.4 µm AIB