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Processing by interstellar photons affects the composition of the icy mantleson interstellar grains. The rate of photodissociation in solids differs fromthat of molecules in the gas phase. The aim of this work was to determine anaverage, general ratio between photodissociation coefficients for molecules inice and gas. A 1D astrochemical model was utilized to simulate the chemicalcomposition for a line of sight through a collapsing interstellar cloud core,whose interstellar extinction changes with time. At different extinctions, thecalculated column densities of icy carbon oxides and ammonia (relative to waterice) were compared to observations. The latter were taken from literature dataof background stars sampling ices in molecular clouds. The best-fit value forthe solid/gas photodissociation coefficient ratio was found to be ~0.3. Inother words, gas-phase photodissociation rate coefficients have to be reducedby a factor of 0.3 before applying them to icy species. A crucial part of themodel is a proper inclusion of cosmic-ray induced desorption. Observationssampling gas with total extinctions in excess of ~22 mag were found to beuncorrelated to modelling results, possibly because of grains being coveredwith non-polar molecules.

The efficiency of photodissociation for molecules in interstellar ices