ON THE FORMATION OF AMINES (RNH2) AND THE CYANIDE ANION (CN-) IN ELECTRON-IRRADIATED AMMONIA-HYDROCARBON INTERSTELLAR MODEL ICES

The present laboratory study simulated cosmic-ray-induced grain chemistry of nitrogen-bearing organic molecules in interstellar and cometary ices. Model ices of ammonia (NH3)-methane (CH4) were prepared and irradiated at 10 K under contamination-free, ultrahigh vacuum conditions with energetic electrons generated in the track of galactic cosmic-ray particles. The radiolysis-induced processing of nitrogen-bearing molecules was then monitored on line and in situ by a Fourier transform infrared spectrometer and a quadrupole mass spectrometer during the irradiation phase and subsequent warm-up phases. The analogous processing was also achieved in ammonia (NH3) and six hydrocarbon (CnH2n+2; n = 1-6) ices. The formation of cyanide anion (CN-) was commonly observed in both ices at 10 K, the temporal column density fit of which traced back the involvement of methylamine (CH3NH2)-based intermediates. Traces of CH3NH2 were evident at about 110 K through thin ammonia matrices in sublimation. From the point of radiative transfer, we further constrain the formationmechanism of aminoacetonitrile (NH2CH2CN) on icy grains of Sgr B-2(N) under a cosmic-ray-induced photon field.