Energetic processing of nitrile-bearing ices by ultraviolet and X-rays radiation

Nitriles represent a quite abundant class of molecules in a variety of astrophysical environments. Acetonitrile (CH3CN), propionitrile (C₂H₅CN) and acrylonitrile (C₂H₃CN) are common examples of saturated and unsaturated organic nitriles present in dark clouds and star-forming regions, while isobutyronitrile (i-C₃H₇CN) was the first branched molecule to be detected in the interstellar medium (ISM). Noteworthy, CH₃CN is a known cometary molecule, and it has been recently detected towards the V4046 Sgr, MWC 480, and HD 163296 protoplanetary disks, what points to the link between the interstellar and nascent planetary systems. However, the reaction routes for producing and destroying such complex molecules are not understood. At temperatures as low as 10 K, gas-phase molecules are expected to condense on dust particles, forming ice mantles. In the case of protoplanetary disks, these ices are exposed to the stellar radiation field from hot young stars, mainly in the far ultraviolet and soft X-ray energy range, what triggers the energetic processing of condensed molecules. In order to estimate the photostability of condensed organic nitriles, we derived the cross-sections for destruction of the parent molecules under conditions that mimic those found in the ISM. The experiments were performed at the SGM beamline at the Brazilian synchrotron laboratory (LNLS). The ices were grown in the multilayer regime, IN-SITU, onto a gold foil substrate cooled at 15 K. Each of these ices was exposed to different doses of zero order synchrotron radiation, and the damage induced after each irradiation was probed by the Near Edge X-ray Absorption Fine Structure (NEXAFS) technique at the N K-edge in the total electron yield (TEY) detection mode. As a general trend, it was observed the decrease of the N1s → π*(CN) excitation at ~399 eV, and its splitting into three main components, due to the different chemical environment of the irradiated ices. The two new spectral features appeared around ~398 and ~401 eV. This result might indicate the synthesis of imine-based compounds (R–C=C=NH) from nitrile-bearing ices exposed to UV and X-ray radiation, corroborating that the abundance of imines, such as the known interstellar ketenimine (CH2CNH), might be correlated to the abundance of nitriles. Cross-sections for the destruction of the parent molecule were derived from the integrated resonance as a function of the photon exposure. The extrapolation to the typical X-ray photon flux from young stellar objects indicates that nitriles should be processed within the first millions of years of activity of the central protostellar. We hope that these parameters may aid in the developing of chemical evolution models of nitrile rich environments in the ISM. (author)