Abstract
A star embedded in a molecular cloud will produce an H II region surrounded by a region of photodissociated atomic gas. We constructed spherical models of the photodissociation (or photon-dominated) region by calculating the balance of photodissociation and formation of H_2. The calculation of the photodissociation rate of H_2 includes the Lyman and Werner transitions from the 256 bound levels of the ground state. Collisions with H and H_2, and quadrupole radiative transitions are included in the calculation of the ground state levels. The UV radiation from the central star is attenuated by dust absorption and self-shielding by H_2.
