This is part 1 of the proposal. It contains the full scientific
 justification etc. and the category I source list. The category II
 and III  source list is contained in part 2 of the proposal. Part
 2 request another 34540 sec observing time, so that the total proposal
 is 72270 sec in each launch window.

 We propose a detailed spectroscopic study of PDRS, selected over a
 broad spectrum of physical parameters beyond that included in the
 central programme. Together with advanced PDR models that account for
 complex but realistic cloud topologies (e.g. clumpiness, physical
 state of the interclump medium, nature of the clump/interclump
 boundary) and for previously neglected, though often important,
 aspects related to the dynamic evolution of the PDR structure (e.g.
 location and propagation of the ionization and the dissociation front
 under different physical conditions), these observations can provide
 important information on the physics governing the interaction of UV
 radiation and the ISM in a wide range of astronomical objects.