We propose to use the LWS to obtain the complete FIR spectra of gas surrounding
a sample of Southern Hemisphere Galactic HII regions.  We shall examine in
detail the effect of the ultraviolet radiation field strengths on the
photodissociation regions (PDRs), the interfaces between the HII regions and the
ambient molecular material.  If these PDRs are to be understood, observations of
many species in different states (ionized, atomic, and molecular) are needed in
order to disentangle the competing effects of far ultraviolet (FUV) radiation
field, source geometry, excitation, and chemistry.  Our scientific goals are to
observe a number of sources located throughout the Milky Way's disk in order to
establish general trends, to observe regions spanning a wide range of UV field
strengths, and, in particular, to compare for the first time global CO, [CII],
[CI], and far infrared (FIR) continuum dust fluxes, which are some of the most
important PDR diagnostics, for a statistically significant source sample.  We
have begun an observational program to study the [CI] line emission at 609
microns from Southern Hemisphere HII regions using the Antarctic Submillimeter
Telescope and Remote Observatory (AST/RO).  AST/RO will also give us CO data
using the 2->1 and 4->3 rotational transitions.  The dust properties will be
gotten using the FIR continuum derived from both ISO and IRAS measurements.
These data will provide the stellar luminosities and extinctions which are both
essential parameters in PDR models.  The unsurpassed sensitivity of ISO will
allow us to detect a number of FIR lines.  The atomic fine structure lines are
PDR diagnostics.  The ionic lines provide a measure of the hardness of the UV
field, HII region densities, and element abundances.  The molecular lines
available to the LWS (H20, CO, OH, and NH3) probe the warm, dense molecular gas
associated with the PDRs.  These data will therefore provide a complete census
of the HII/PDR/GMC ISM in all of its phases which can be used to determine the
effect of UV field strength on the ambient gas, to test the predictions of
theoretical PDR models, and to establish whether PDR properties change
systematically across the galactic disk.