Interstellar chemistry - Gas and surface phase processes; time dependence

The astrophysical information content in data on interstellar molecules may be tapped by studying chemical models of interstellar clouds. The considerations to be made in setting up such a model are briefly described. Such models do not lead to unique descriptions, and a procedure useful in minimizing the ambiguities in some diffuse cloud models is given. Dense clouds also require careful treatment. However, the situation in them is complicated by many time-dependent factors, including especially the problem of accretion of gas phase molecules onto grain surfaces. Uncertainties in the processes of adsorption and desorption, and in the dynamics of dense clouds, currently prevent a clear description of these astronomical objects.