  We examine the formation of molecular gas behind shocks in atomic gas using a
chemical/dynamical model, particular emphasis is given to constraints the
chemistry places on the dynamical evolution. The most important result of this
study is to stress the importance of shielding the molecular gas from the
destructive effects of UV radiation. For shock ram pressures comparable to or
exceeding typical local ISM pressures, self-shielding controls the formation
time of H2 but CO formation requires shielding of the interstellar radiation
field by dust grains. We find that the molecular hydrogen fractional abundance
can become significant well before CO forms. The timescale for (CO) molecular
cloud formation is not set by H2 formation, but rather by the timescale for
accumulating a sufficient column density or extinction, A_V > 0.7. The local
ratio of atomic to molecular gas (4:1), coupled with short estimates for cloud
lifetimes (3-5 Myr), suggests that the timescales for accumulating molecular
clouds from atomic material typically must be no longer than about 12-20 Myr.
Based on the shielding requirement, this implies that the typical product of
pre-shock density and velocity must be n*v > 20 cm^-3 km s^-1. Based on these
results we find that flow-driven formation of molecular clouds in the local
interstellar medium can occur sufficiently rapidly to account for observations.
We also provide detailed predictions of atomic and molecular emission and
absorption that track molecular cloud formation, with a view toward helping to
verify cloud formation by shock waves. Finally, we provide an analytic solution
for time-dependent H2 formation which may be of use in numerical hydrodynamic
calculations.
