2000ApJ...534..801P

We have investigated the effects induced by ion bombardment of mixtures containing nitrogen-bearing compounds at low temperatures. The results show the formation of a band at 2080 cm^–1 in binary mixtures, NH₃:CH₄ and N₂:CH₄, which we attribute to HCN embedded in the organic residue formed by ion irradiation. In addition to this band, ternary mixtures containing an oxygen-bearing species (i.e., H₂O) form a compound with a prominent absorption band at about 2165 cm^–1 (4.62 µm). We ascribe this band to a nitrile compound containing O that is bonded to the organic residue. A detailed comparison of the laboratory results with astronomical data of the 4.62 µm absorption band in protostellar spectra shows good agreement in peak position and profile. Our experimental studies show that N₂, which is a more likely interstellar ice component than NH₃, can be the molecular progenitor of the carrier of the interstellar band. This is an alternative to the pathway by which UV photolysis of NH₃-containing ices produces the 4.62 µm band and implies that ion bombardment may well play an important role in the evolution of interstellar ices. Here, we discuss the implications of our studies for the chemical route by which the carrier of the 4.62 µm band is formed in these laboratory experiments.