R-O-C N species produced by ion irradiation of ice mixtures: Comparison with astronomical observations

We have investigated the effects induced by ion bombardment of mixtures con taining nitrogen-bearing compounds at low temperatures. The results show th e formation of a band at 2080 cm(-1) in binary mixtures, NH3:CH4 and N-2:CH 4, which we attribute to HCN embedded in the organic residue formed by ion irradiation. In addition to this band, ternary mixtures containing an oxyge n-bearing species (i.e., H2O) form a compound with a prominent absorption b and at about 2165 cm(-1) (4.62 mu m). We ascribe this band to a nitrile com pound containing O that is bonded to the organic residue. A detailed compar ison of the laboratory results with astronomical data of the 4.62 pm absorp tion band in protostellar spectra shows good agreement in peak position and profile. Our experimental studies show that N-2, which is a more likely in terstellar ice component than NH3, can be the molecular progenitor of the c arrier of the interstellar band. This is an alternative to the pathway by w hich UV photolysis of NH3-containing ices produces the 4.62 mu m band and i mplies that ion bombardment may well play an important role in the evolutio n of interstellar ices. Here, we discuss the implications of our studies fo r the chemical route by which the carrier of the 4.62 mu m band is formed i n these laboratory experiments.