  CO2 ice is one of the most abundant components in ice-coated interstellar
ices besides H2O and CO, but the most favorable path to CO2 ice is still
unclear. Molecular dynamics calculations on the ultraviolet photodissociation
of different kinds of CO-H2O ice systems have been performed at 10 K in order
to demonstrate that the reaction between CO and an OH molecule resulting from
H2O photodissociation through the first excited state is a possible route to
form CO2 ice. However, our calculations, which take into account different ice
surface models, suggest that there is another product with a higher formation
probability ((3.00+-0.07)x10-2), which is the HOCO complex, whereas the
formation of CO2 has a probability of only (3.6+-0.7)x10-4. The initial
location of the CO is key to obtain reaction and form CO2: the CO needs to be
located deep into the ice. The HOCO complex becomes trapped in the cold ice
surface in the trans-HOCO minimum because it quickly loses its internal energy
to the surrounding ice, preventing further reaction to H + CO2. Several
laboratory experiments have been carried out recently and they confirm that CO2
can also be formed through other, different routes. Here we compare our
theoretical results with the data available from experi- ments studying the
formation of CO2 through a similar pathway as ours, even though the initial
conditions were not exactly the same. Our results also show that the HCO van
der Waals complex can be formed through the interaction of CO with the H atom
that is formed as a product of H2O photodissociation. Thus, the reaction of the
H atom photofragment following H2O photodissociation with CO can be a possible
route to form HCO ice.
