Production of NH4+ and OCN- ions by the interaction of heavy-ion cosmic rays with CO-NH3 interstellar ice

In the interstellar medium, high-energy cosmic rays are able to penetrate deeply into molecular clouds, resulting in the ionization and dissociation of condensed molecules on ice-grain surfaces, which in turn leads to the desorption of a number of species. The interpretation of interstellar data requires qualitative and quantitative studies of the molecular species formed in several processes. In this work, the interaction of cosmic rays with CO-NH3 ice is simulated by the bombardment of similar to 65-MeV heavy ions (masses of similar to 100 u) on frozen gases. Positive and negative ions ejected from the ice surface as a consequence of the impact are analysed through time-of-flight mass spectrometry. The dependence of the ion desorption yield on the sample temperature (which varies/increases with time) has been used to discuss the emission process of the desorbed ions, especially near ice sublimation temperatures (similar to 30 K for CO at 10(-8) mbar); at these temperatures, HCO+, NO+, NH3OH+ and NH3CO+ are formed and emitted. For temperatures in the 40 to 65 K range, mass spectra of positive ions are dominated by the NH4+ ion peak, similarly to what occurs for a pure frozen NH3 target. Concerning the mass spectra of negative ions, the total yield of negative ions at T = 25 K is found to be about 20 times lower than that of positive ions. Before CO sublimation, CN-bearing species are formed, among which the cyanide ions CN- and OCN- are by far the most abundant species. These findings support the idea that NH4+ and OCN- ions are efficiently formed by the interaction of cosmic rays with CO-NH3 ices.