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==>  This proposal requests an upgrade from Priority 3 for DNEUFELD.HHALIDES
(Please note that the scientific justification and proposed observing programme
have been modified in response to developments since the pre-launch call.)
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We propose to search for far-infrared transitions of interstellar hydrogen
chloride and hydrogen fluoride.  We expect the proposed observations to serve
as a valuable probe of the chemistry of halogen-bearing interstellar molecules.
For hydrogen chloride, we will search for the far-infrared J=3-2 rotational
transition in two sources: Sagittarius B2, in which the submillimeter J=1-0 line
of HCl has recently been detected in absorption using the KAO; and the Orion
Molecular Cloud 1 (OMC-1), in which the J=1-0 line has been detected in emission
and has recently been mapped.  Based upon theoretical models for the excitation
of HCl J=1-0 in these sources, we expect the J=3-2 transitions to be easily
detectable with ISO.  Measurements of the higher-lying J=3-2 line will serve
to constrain the excitation conditions in the source and will allow the HCl
abundance to be determined more reliably.  For hydrogen fluoride, we will search
for the J=2-1 rotational transition in the same two sources.   A very recent
ground-based spectral line survey of the Orion region has led to the detection
of a single line that has been tentatively identified as the J=1-0 transition of
deuterium fluoride (DF); unfortunately, the analogous transition of HF cannot be
detected from the ground because of atmospheric absorption, so space-based
observations will be needed to search for hydrogen fluoride.  If the
identification of DF in Orion is correct, then the depletion of fluorine must
be small and HF must be a major reservoir of gas-phase fluorine.  A detection of
HF J=2-1 would mean the first unequivocal detection of a fluorine-bearing
interstellar molecule.