  Processing by interstellar photons affects the composition of the icy mantles
on interstellar grains. The rate of photodissociation in solids differs from
that of molecules in the gas phase. The aim of this work was to determine an
average, general ratio between photodissociation coefficients for molecules in
ice and gas. A 1D astrochemical model was utilized to simulate the chemical
composition for a line of sight through a collapsing interstellar cloud core,
whose interstellar extinction changes with time. At different extinctions, the
calculated column densities of icy carbon oxides and ammonia (relative to water
ice) were compared to observations. The latter were taken from literature data
of background stars sampling ices in molecular clouds. The best-fit value for
the solid/gas photodissociation coefficient ratio was found to be ~0.3. In
other words, gas-phase photodissociation rate coefficients have to be reduced
by a factor of 0.3 before applying them to icy species. A crucial part of the
model is a proper inclusion of cosmic-ray induced desorption. Observations
sampling gas with total extinctions in excess of ~22 mag were found to be
uncorrelated to modelling results, possibly because of grains being covered
with non-polar molecules.
