We propose to follow-up our recent discovery of interstellar hydrogen fluoride
with a search for the J=2-1 line of HF in two additional sources: Orion
Molecular Cloud 1 (OMC-1) and W51.  Previously, we have searched for hydrogen
fluoride in the single source Sagittarius B2, and obtained a detection of the
HF J=2-1 absorption line at 121.6973 microns (Neufeld et al. 1997, ApJ,
488, L141). This detection marks the first discovery of a fluorine molecule in
an interstellar gas cloud and the first time that a new interstellar molecule
has been detected at far-infrared wavelengths.  The derived HF abundance of
3E-10 inferred from this observation implied that the element fluorine shows a
gas-phase depletion factor ~ 50 along the line-of-sight to Sgr B2, given
reasonable assumptions about the fluorine chemistry and the total elemental
abundance of fluorine.  The amount of ISO time previously awarded for this
project (4000 sec) did not permit us to observe any source other than Sgr B2.
In this request for ISO discretionary time, we propose to follow up the
discovery of HF with observations of two other sources that are very bright
at far-infrared wavelengths: OMC-1 and W51.  By observing hydrogen fluoride
in these additional sources and obtaining measurements of (or upper limits on)
its  abundance, we will obtain valuable constraints upon the fluorine
chemistry  and fluorine depletion in a variety of interstellar environments
within the Galaxy.  Because hydrogen fluorine possesses one of the largest
rotational constants of any molecule, its rotational spectrum lies entirely
shortward of  the atmospheric windows within which most molecules show
rotational transitions.  Ground-based observations of interstellar HF are
therefore impossible, and ISO provides the last opportunity for many years to
follow-up our recent discovery of HF.