Cosmic ray processing of N<SUB>2</SUB>-containing interstellar ice analogues at dark cloud conditions

Abstract

N<SUB>2</SUB> is believed to lock considerable part of nitrogen elemental budget and, therefore, to be one of the most abundant ice constituent in cold dark clouds. This laboratory-based research utilizes high energetic processing of N<SUB>2</SUB> containing interstellar ice analogues using 200 keV H<SUP>+</SUP> and He<SUP>+</SUP> ions that mimics cosmic ray processing of the interstellar icy grains. It aims to investigate the formation of (iso)cyanates and cyanides in the ice mantles at the conditions typical for cold dark clouds and prestellar cores. Investigation of cosmic ray processing as a chemical trigger mechanism is explained by the high stability of N<SUB>2</SUB> molecules that are chemically inert in most of the atom- and radical-addition reactions and cannot be efficiently dissociated by cosmic ray induced UV-field. Two sets of experiments are performed to closer address solid-state chemistry occurring in two distinct layers of the ice formed at different stages of dark cloud evolution, i.e. `H<SUB>2</SUB>O-rich' and `CO-rich' ice layers. Formation of HNCO and OCN<SUP>-</SUP> is discussed in all of the performed experiments. Corresponding kinetic curves for HNCO and OCN<SUP>-</SUP> are obtained. Furthermore, a feature around 2092 cm<SUP>-1</SUP> assigned to the contributions of <SUP>13</SUP>CO, CN<SUP>-</SUP>, and HCN is analysed. The kinetic curves for the combined HCN/CN<SUP>-</SUP> abundance are derived. In turn, normalized formation yields are evaluated by interpolation of the obtained results to the low irradiation doses relevant to dark cloud stage. The obtained values can be used to interpret future observations towards cold dark clouds using James Webb Space Telescope.