Photo-processing of astro-PAHs

TL;DR

This study investigates the behavior of large astro-PAHs under VUV irradiation, revealing ionization dominance and a minor hydrogen dissociation channel, which impacts their stability in astrophysical environments.

Contribution

It provides experimental insights into the molecular properties and fragmentation pathways of large PAHs, especially C48H20+ , under VUV processing, informing astrophysical models.

Findings

Ionization dominates over fragmentation in large PAHs.

Hydrogen dissociation occurs via excited electronic states.

C2H2 is not a significant fragmentation channel.

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are key species in astrophysical environments in which vacuum ultraviolet (VUV) photons are present, such as star-forming regions. The interaction with these VUV photons governs the physical and chemical evolution of PAHs. Models show that only large species can survive. However, the actual molecular properties of large PAHs are poorly characterized and the ones included in models are only an extrapolation of the properties of small and medium-sized species. We discuss here experiments performed on trapped ions including some at the SOLEIL VUV beam line DESIRS. We focus on the case of the large dicoronylene cation, C48H20+ , and compare its behavior under VUV processing with that of smaller species. We suggest that C2H2 is not a relevant channel in the fragmentation of large PAHs. Ionization is found to largely dominate fragmentation. In addition, we report evidence for a hydrogen dissociation channel through excited electronic states. Although this channel is minor, it is already effective below 13.6 eV and can significantly influence the stability of astro-PAHs. We emphasize that the competition between ionization and dissociation in large PAHs should be further evaluated for their use in astrophysical models.