  Current gas phase models do not account for the abundances of HNCO isomers
detected in various environments, suggesting a formation in icy grain mantles.
We attempted to study a formation channel of HNCO and its possible isomers by
vacuum-UV photoprocessing of interstellar ice analogues containing H$_2$O,
NH$_3$, CO, HCN, CH$_3$OH, CH$_4$, and N$_2$ followed by warm-up, under
astrophysically relevant conditions. Only the H$_2$O:NH$_3$:CO and H$_2$O:HCN
ice mixtures led to the production of HNCO species. The possible isomerization
of HNCO to its higher energy tautomers following irradiation or due to ice
warm-up has been scrutinized. The photochemistry and thermal chemistry of
H$_2$O:NH$_3$:CO and H$_2$O:HCN ices was simulated using the Interstellar
Astrochemistry Chamber (ISAC), a state-of-the-art ultra-high-vacuum setup. The
ice was monitored in situ by Fourier transform mid-infrared spectroscopy in
transmittance. A quadrupole mass spectrometer (QMS) detected the desorption of
the molecules in the gas phase. UV-photoprocessing of
H$_2$O:NH$_3$:CO/H$_2$O:HCN ices lead to the formation of OCN$^-$ as main
product in the solid state and a minor amount of HNCO. The second isomer HOCN
has been tentatively identified. Despite its low efficiency, the formation of
HNCO and the HOCN isomers by UV-photoprocessing of realistic simulated ice
mantles, might explain the observed abundances of these species in PDRs, hot
cores, and dark clouds.
