Effect of protonation on the UV/VUV photostability of cyano-substituted anthracene and phenanthrene

Context. The vacuum ultraviolet (VUV) photoprocessing of polycyclic aromatic hydrocarbons (PAHs) has been established as a key piece of the puzzle to understand the life cycle of carbon-based molecules in space. The recent detection of cyano (CN) aromatic species, with unexpectedly high abundance, motivated the current study of investigating their interaction with UV/VUV radiation.Aims. The aims were to investigate the fate, after VUV photoexcitatation, of medium-size (three rings) CN-PAH radical cations and of their protonated analogs, and thus to assess the effect of protonation on the photostability of the CN-PAHs. Photoproducts (ionic fragments and dications) were mass-analyzed and measured as a function of photon energy. The results were also compared with those for the bare anthracene radical cation to assess the influence of the added CN group.Methods. The positively charged CN-PAHs were stored in a quadrupole ion trap prior to interrogation by UV/VUV radiation, with photon energies between 4.5 and 13.6 eV, delivered by the DESIRS beamline from the synchrotron SOLEIL.Results. The HCN/HNC loss channel is present for both radical cations and protonated species, but H-2 loss is only apparent for the radical cations. Based on comparison with quantum chemical calculations, radiative and/or collisional processes should be relevant at energies lower than 8 eV, with a stronger propensity for radical cation than protonated CN-PAHs. The cata-condensed 9-CN-anthracene has a nearly two-fold larger photoionization yield at 13.6 eV than peri-condensed 9-CN-phenanthrene.Conclusions. The photoionization yield of singly and doubly ionized CN-PAHs is greater for radical cations than for protonated analogs. The photoionization yields of CN-PAHs is diminished by protonation and, in the future, similar investigations should target larger protonated CN-PAHs to support a general model for the photo-processing of these relevant molecular systems. Similar processes to those for the bare PAH radical cations may involve the radical cations of CN-PAHs, making their addition important in models that describe the photoelectric heating of interstellar gas.