Dissociation-ionization fronts in interstellar gas clouds-I. Physical processes

The effects of ultraviolet radiation from early type stars on clouds of molecular hydrogen are considered. The reactions taking place within the front are discussed and it is concluded that whichever way the reactions proceed the effective transition is: H2 2H 2H+ + 2e. The dissociation and ionization take place within a single thin region which we call a dissociation-ionization front. Microscopic processes within the dissociation-ionization front are investigated and it is shown that, while good thermalization exists between the translational and rotational modes of the different species, the vibrational mode of the molecules remain `frozen' throughout the front. Consequently collisional dissociation within the front is insignificant. The effects, of radiation pressure and of magnetic and gravitational forces, are seen to be unimportant in the treatment of the fronts. This paper clears the ground for a subsequent one on the structure and the propagation of dissociation-ionization fronts in interstellar gas clouds containing molecular hydrogen.