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 (NH 3 )–methane (CH 4 ) 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 (NH 3 ) and six hydrocarbon (C n H 2 n +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 (CH 3 NH 2 )-based intermediates. Traces of CH 3 NH 2 were evident at about 110 K through thin ammonia matrices in sublimation. From the point of radiative transfer, we further constrain the formation mechanism of aminoacetonitrile (NH 2 CH 2 CN) on icy grains of Sgr B2(N) under a cosmic-ray-induced photon field.