UV/VUV photoprocessing of protonatedN-hetero(poly)acenes

Abstract

ABSTRACTN-heterocycles are suspected to play an important role in the chemical origin of life. Despite their detection in meteorites and in Titan’s atmosphere, their extra-terrestrial chemical formation networks remain elusive. Furthermore N-heterocyclics are undetected in the interstellar medium. This paper assesses the photostability of protonated N-hetero(poly)acenes after ultraviolet (UV) and vacuum ultraviolet (VUV) excitation. It provides information on their ability to retain the N atom into the cycle to generate larger N-containing species or functionalized N-heterocyles. Protonated N-hetero(poly)acenes were generated using electrospray ionization and injected into a linear ion trap where they were irradiated by radiation of 4.5 to 10 eV using the DESIRS beamline at the synchrotron SOLEIL. The photodissociation action spectra of protonated pyridine, quinoline, isoquinoline, and acridine were measured by recording the photofragment yields as a function of photon energy. The four systems exhibit dissociation channels associated with H2 and HCN/HNC loss but with different branching ratios. The results indicate that increasing the size of the N-hetero(poly)acenes increases the chance of retaining the N atom in the larger fragment ion after photodissociation but it remains that all the protonated N-hetero(poly)acenes studied lose their N atom at part of a small neutral photofragment, with high propensity. Therefore, protonated N-hetero(poly)acenes in interstellar space are unlikely precursors to form larger N-containing species. However, protonated pyridine, quinoline, isoquinoline, and acridine are most likely to retain their N atoms in planetary atmospheres where UV radiation at the planet’s surface is typically restricted to wavelengths greater than 200 nm – suggesting such environments are possible substrates for prebiotic chemistry.