Using LWS guaranteed time in the core program HSMITH_IRBS, we obtained LWS01
spectra from 43 to 196 um of the nuclei of the nearby starburst galaxies, M82
and NGC 253.  The M82 spectrum contains the expected 7 bright far-IR fine
structure lines, but also contains obvious absorption lines at 119.3 and 149.2
um that we identify as the fundamental rotational transitions of OH and CH
respectively.  There also are probable detections of the HD rotational lines in
emission at 112.1 um (J=1-0), and 56.2 um (J=2-1).  These molecular rotational
lines are apparent in the NGC 253 spectrum as well.  We propose to use the LWS
Fabry-Perot to spectrally resolve the 119, 149 and 112 um lines, and the LWS to
get ten times better S/N on the 56 um line for both galaxies, thereby confirming
the line identifications.  Each of these lines can be used for abundance
analyses.  The CH and OH absorption lines lead directly column density, hence
abundance, thereby testing the chemical models of molecular clouds exposed to
the very large (~ 10^3 ISRF) far-UV radiation fields, and high cosmic ray fluxes
associated with the starburst.  We will compared the observed far-IR OH line
profiles to radio observations of the 18 cm line to determine the excitation
temperature of the radio transitions.  Most of the radio profile is in
absorption, so that the excitation temperature is likely largely determined by
collisional processes.  The line comparison will therefore yield molecular cloud
densities as a function of radial velocity.  For M82 at a LSR velocity ~ 75 km
s^-1, however, the 18 cm lines are in emission, and likely masing, so that
determining the excitation temperature connects to models of the pumping scheme
for masers.
Confirming observations of the HD lines is especially exciting.  The pair of
lines yields the physical parameters of the emitting gas, hence column density
of HD along the line of sight.  Since D is only destroyed by normal stellar
process, the observed D abundance is a lower limit to the primordial D
abundance, hence upper limit to the baryon density of the Universe.  Our initial
line analysis for M82 indicates the HD lines come from gas at T ~ 120 K, and
that the D/H ratio ~ 1.6 E-5.