Formation of N-bearing complex organic molecules in molecular clouds: Ketenimine, acetonitrile, acetaldimine, and vinylamine via the UV photolysis of C2H2 ice

Context. The solid-state C2H2 chemistry in interstellar H2O-rich ice has been proposed to explain astronomically observed complex organic molecules (COMs), including ketene (CH2CO), acetaldehyde (CH3CHO), and ethanol (CH3CH2OH), toward early star-forming regions. This formation mechanism is supported by recent laboratory studies and theoretical calculations for the reactions of C2H2+OH/H. However, the analog reaction of C2H2+NH2 forming N-bearing species has been suggested to have a relatively low rate constant that is orders of magnitude lower than the value of C2H2+OH. Aims. This work extends our previous laboratory studies on O-bearing COM formation to investigate the interactions between C2H2 and NH3 ice triggered by cosmic ray-induced secondary UV photons under molecular cloud conditions. Methods. Experiments were performed in an ultra-high vacuum chamber to investigate the UV photolysis of the C2H2:NH3 ice mixture at 10 K. The ongoing chemistry was monitored in situ by Fourier-transform infrared spectroscopy as a function of photon fluence. The IR spectral identification of the newly formed N-bearing products was further secured by a quadrupole mass spectrometer during the temperature-programmed desorption experiment. Results. The studied ice chemistry of C-2 H-2 with NH2 radicals and H atoms resulting from the UV photodissociation of NH3 leads to the formation of several N-bearing COMs, including vinylamine (CH2CHNH2), acetaldimine (CH3CHNH), acetonitrile (CH3CN), ketenimine (CH2CNH), and tentatively ethylamine (CH3CH2NH2). The experimental results show an immediate and abundant CH2CHNH2 yield as the first-generation product, which is further converted into other chemical derivatives. The effective destruction and formation cross-section values of parent species and COMs were derived, and we discuss the chemical links among these molecules and their astronomical relevance.