  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.
