MECHANISTICAL STUDIES ON THE FORMATION AND NATURE OF THE “XCN” (OCN–) SPECIES IN INTERSTELLAR ICES

摘要

We conducted laboratory experiments on the interaction of ionizing radiation in the form of energetic electrons with interstellar model ices to investigate the nature and possible routes to form the "XCN" species as observed at 4.62 μm (2164?cm–1) in the interstellar medium. Our laboratory experiments provided compelling evidence that the isocyanide ion (OCN–) presents the carrier of the "XCN" feature in interstellar ices. Most importantly, the studies exposed—based on kinetic fits of the temporal profiles of important reactants, intermediates, and products—that two formation mechanisms can lead to the production of the isocyanide ion (OCN–) in low-temperature interstellar ices. In carbon monoxide-ammonia ices, unimolecular decomposition of ammonia leads to reactive NH2 and NH radical species, which in turn can react with neighboring carbon monoxide to form ultimately the isocyanide ion (OCN–); this process also involves a fast proton transfer to a base molecule in the surrounding ice. Second, cyanide ions (CN–)—formed via unimolecular decomposition of methylamine (CH3NH2) via a methanimine (CH2NH) intermediate—can react with suprathermal oxygen atoms forming the isocyanide ion (OCN–). We also discuss that the isocyanide ion (OCN–) can be used as a molecular tracer to determine, for instance, the development stage of young stellar objects and also the chemical history of ices processed by ionizing radiation.