  Complex organic molecules (COMs) can be produced by energetic processing of
interstellar ice mantles accreted on top of dust grains. Two COMs with proposed
energetic ice formation pathways are formamide and acetaldehyde. Both have been
detected in Solar System comets, and in different circumstellar and
interstellar environments. In this work, we study the NH2CHO and CH3CHO
formation upon UV photoprocessing of CO:NH3 and CO:CH4 ice samples. The
conversion from NH2 radicals to NH2CHO is 2-16 times higher than the conversion
from CH3 radicals to CH3CHO under the explored experimental conditions, likely
because the formation of the latter competes with the formation of larger
hydrocarbons. In addition, the conversion of NH2 into NH2CHO at 10 K increases
with the NH3 abundance in the ice, and also with the temperature in
CO-dominated CO:NH3 ices. This is consistent with the presence of a small NH2
and HCO reorientation barrier for the formation of NH2CHO, which is overcome
with an increase in the ice temperature. The measured NH2CHO and CH3CHO
formation efficiencies and rates are similar to those found during electron
irradiation of the same ice samples under comparable conditions, suggesting
that both UV photons and cosmic rays would have similar contributions to the
solid-state formation of these species in space. Finally, the measured
conversion yields (up to one order of magnitude higher for NH2CHO) suggest that
in circumstellar environments, where the observed NH2CHO/CH3CHO abundance ratio
is 0.1, there are likely additional ice and/or gas phase formation pathways for
CH3CHO.
