Formation of N-bearing complex organic molecules in molecular clouds: Ketenimine, acetonitrile, acetaldimine, and vinylamine via the UV photolysis of C$_2$H$_2$ ice

Abstract: The solid-state C$_2$H$_2$ chemistry in interstellar H$_2$O-rich ice has been proposed to explain astronomically observed complex organic molecules (COMs), including ketene (CH$_2$CO), acetaldehyde (CH$_3$CHO), and ethanol (CH$_3$CH$_2$OH), toward early star-forming regions. This formation mechanism is supported by recent laboratory studies and theoretical calculations for the reactions of C$_2$H$_2$+OH/H. However, the analog reaction of C$_2$H$_2$+NH$_2$ forming N-bearing species has been suggested to have a relatively low rate constant that is orders of magnitude lower than the value of C$_2$H$_2$+OH. This work extends our previous laboratory studies on O-bearing COM formation to investigate the interactions between C$_2$H$_2$ and NH$_3$ ice triggered by cosmic ray-induced secondary UV photons under molecular cloud conditions. Experiments were performed in an ultra-high vacuum chamber to investigate the UV photolysis of the C$_2$H$_2$:NH$_3$ ice mixture at 10 K. The studied ice chemistry of C$_2$H$_2$ with NH$_2$ radicals and H atoms resulting from the UV photodissociation of NH$_3$ leads to the formation of several N-bearing COMs, including vinylamine (CH$_2$CHNH$_2$), acetaldimine (CH$_3$CHNH), acetonitrile (CH$_3$CN), ketenimine (CH$_2$CNH), and tentatively ethylamine (CH$_3$CH$_2$NH$_2$). The experimental results show an immediate and abundant CH$_2$CHNH$_2$ 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.