Diffuse and translucent molecular clouds provide rigorous tests of
theories of interstellar chemistry and of cloud structure because they
can be observed both through UV/visible/IR absorption lines and radio-
frequency emission lines. We propose to fill in some crucial missing
information through ISO spectroscopy on IR emission lines of [C II],
[O I], and H_2.  These data will provide direct constraints on the
cooling and thermal structure of these molecular clouds, which are thin
enough that their chemistry and ionization are mostly photon-dominated.
Most of these clouds are thought not to be bound by their own self-
gravity, which means that effects of pressure are significant. Moreover
their low densities, 100-1000 cm^-3, imply that one is more likely to
see observable manifestations of turbulence on their chemistry and
thermal structure than in the denser clouds where turbulence is damped
out on smaller scales.
   Our target list has been selected according to two criteria.  First,
the targets are all clouds for which we have already obtained extensive
ground-based data on optical absorption lines and mm-wave emission
lines.  Second, the lines of sight to these clouds are relatively
uncontaminated by foreground or background matter and the observed line
profiles are typically simple.  Even so, the target list includes clouds
with a variety of properties:  clouds closely associated with shocks,
isolated clouds, clouds whose abundances are primarily atomic, and
clouds with well developed molecular abundances.
   The observations will be interpreted with reference to detailed
theoretical models.  The ISO data will provide direct tests of our
understanding of the thermal structure and of some aspects of the
chemistry of photon-dominated regions (PDRs), of which our clouds are
simple examples.