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A molecular hydrogen dissociation front (H2 dissociation front or in context, just dissociation front) is a boundary within a cloud or atmosphere between molecular and atomic hydrogen. If lower depths (inner portions) of a cloud or atmosphere are hotter, then the dissociation front constitutes the boundary around the volume sufficiently hot to dissociate hydrogen molecules. The boundary can have a significant impact on opacity, thus marks a difference in cooling ability and the length of its cooling time, and potentially more, such as whether the gas is convective or radiative. In the case of planet formation, it is a factor in the ultimate amount of atmosphere retained by the planet.
An ionization front is an analogous boundary between gas hot enough to remain ionized versus that remaining neutral.