New models of interstellar gas-grain chemistry - II. Surface photochemistry in quiescent cores

The photodissociation of surface species, caused by photons from the cosmic-ray-induced and background interstellar radiation fields, is incorporated into our combined gas-phase and grain-surface chemical models of quiescent dense interstellar cores. For the cores studied here, only cosmic-ray-induced photons are important. We find that photodissociation alters gas-phase and surface abundances mainly at large cloud ages (greater than or similar to 10(6-7) yr), The abundances of those surface species, such as H2O, that are readily reproduced on the surface following photodissociation are not strongly affected at any time. The abundances of surface species that are, on the other hand, reformed slowly via surface reactions possessing activation energy (e.g. CH3OH) are reduced, while the abundances of associated surface photoproducts (e.g, CO) increase. In the gas phase, inclusion of surface photodissociation tends to increase molecular abundances at late times, slightly improving the agreement with observation for TMC-1.