Vacuum ultraviolet photo-processing of polycyclic aromatic hydrocarbons of astrochemical interest Polycyclic aromatic hydrocarbons (PAHs) absorb the vacuum ultraviolet (VUV) photons emitted by hot are abundant molecules (/home/webapps/asp_fr/data/asp/publications/synchrotron-soleil/synchrotron-soleil-2020/soleil-highlights-2020-hd-ss-tc10% of the stars and subsequently undergo different relaxation processes: interstellar carbon budget) in regions of star photoionization resulting in the emission of photoelectrons, photodissociation resulting in the ejection of small fragments formation where they are photo-processed (mainly H, H, CH), and radiative cooling comprising emission2 2 2 by vacuum ultraviolet (VUV) photons. This in the aromatic infrared bands (AIBs) observed in many astronomical objects. All these relaxation processes, which interaction produces photo-electrons which are summarized in [1] and Figure 1 govern the evolution of the heat the gas by collisions. In this study, astro-PAH population and impact the physics and chemistry trapped PAH cations (sizes of 30–48 carbon of the gas. Chemical models suggest that large PAHs with a atoms) were submitted to VUV photons typical number of carbon atoms of 50 or more dominate the AIB emission and that these species are found in different ionization in the range of 9–20 eV from the DESIRS stages, mainly neutrals and cations [2]. beamline. Ionization cross sections could be indirectly obtained and, for the first time, EXPERIMENTAL METHOD AND RESULTS the photoionization yield of PAH cations The experiments were performed at the DESIRS beamline at SOLEIL, which provided us with intense, coherent, and tunable light and its evolution with molecular size. New in the VUV range. We used the linear ion trap quadrupole mass molecular data are provided to be included spectrometer (SRMS2), in order to trap large PAH cations. The in models describing the physics and production of PAH cations was performed using an atmospheric pressure photoionization source which required the species of chemistry of gas in VUV-irradiated regions. interest to be in solution before their injection with a syringe. This part was a major limitation on the size range of PAHs that FIGURE 1 we were able to study due to the insolubility of large PAHs. We were able to investigate four PAH cations ranging in size from 30 to 48 carbon atoms; namely (a) benzobisanthene, CH,+30 14 (b) ovalene, CH, (c) dibenzophenanthropentaphene (DBPP),+32 14 CH, and (d) dicoronylene, CH. Their molecular structures+ +36 4818 20 are depicted on the left in Figure 2. The trapped PAH cations were energized by VUV photons in the range of 9 to 20 eV and all resulting photoproducts (fragments and dications) were mass-analyzed and recorded as a function of photon energy. This enabled us to quantify the branching ratio (BR) between the two relaxation processes of photoionization (dications) and photodissociation (fragments) as depicted in Figure 2. The BR was found to increase significantly with increasing number of carbon atoms with a maximal value of 0.98 in the case of the largest studied PAH cation, dicoronylene. Therefore, for these INTERSTELLAR PAHS large PAHs, photoionization was found to be the dominant Polycyclic aromatic hydrocarbons (PAHs) play a major role in channel and photodissociation was found to be less important, the physics and chemistry of the interstellar medium (ISM) indemonstrating the increased stability of large PAH species in general, and star forming regions in particular. They stronglyVUV-irradiated astrophysical environments. 62