In order to study the physical conditions in low-excitation photodissociation
regions we propose a detailed spectroscopic study of two bright H2 emission
knots in the "prototypical" reflection nebula NGC 2023.  A first analysis of
our SWS observations of the S140 PDR (Timmermann et al. 1996, attached)
motivates us to also request followup observations of S140 to clarify questions
raised regarding the gas temperature, the ortho-para ratio, and the iron
abundance.  To comparatively study PDRs from significantly harder radiation
fields and the effects of rapidly propagating ionization fronts on the PDR
structure we propose to observe several molecular clumps illuminated by early
type O stars. For this we selected targets in the W5 and Rosette HII regions.