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Understanding the molecular content of galaxies is a critical problem in starformation and galactic evolution. Here we present a new method, based on aStromgren-type analysis, to calculate the amount of HI that surrounds amolecular cloud irradiated by an isotropic radiation field. We consider bothplanar and spherical clouds, and H_2 formation either in the gas phase orcatalyzed by dust grains. Under the assumption that the transition from atomicto molecular gas is sharp, our method gives the solution without any referenceto the photodissociation cross section. We test our results for the planar caseagainst those of a PDR code, and find typical accuracies of about 10%. Ourresults are also consistent with the scaling relations found in Paper I of thisseries, but they apply to a wider range of physical conditions. We presentsimple, accurate analytic fits to our results that are suitable for comparisonto observations and to implementation in numerical and semi-analytic models.

The Atomic-to-Molecular Transition in Galaxies. III. A New Method for Determining the Molecular Content of Primordial and Dusty Clouds