  H2 formation is an important process in post-shock regions, since H2 is an
active participant in the cooling and shielding of the environment. The onset
of H2 formation therefore has a strong effect on the temperature and chemical
evolution in the post shock regions. We recently developed a model for H2
formation on a graphite surface in warm conditions. The graphite surface acts
as a model system for grains containing large areas of polycyclic aromatic
hydrocarbon structures. Here this model is used to obtain a new description of
the H2 formation rate as a function of gas temperature that can be implemented
in molecular shock models. The H2 formation rate is substantially higher at
high gas temperatures as compared to the original implementation of this rate
in shock models, because of the introduction of H atoms which are chemically
bonded to the grain (chemisorption). Since H2 plays such a key role in the
cooling, the increased rate is found to have a substantial effect on the
predicted line fluxes of an important coolant in dissociative shocks [O I] at
63.2 and 145.5 micron. With the new model a better agreement between model and
observations is obtained. Since one of the goals of Herschel/PACS will be to
observe these lines with higher spatial resolution and sensitivity than the
former observations by ISO-LWS, this more accurate model is very timely to help
with the interpretation of these future results.
