  We present a theoretical investigation of the chemistry of fluorine-bearing
molecules in diffuse and dense interstellar gas clouds. The chemistry of
interstellar fluorine is qualitatively different from that of any other
element, because - unlike the neutral atoms of any other element found in
diffuse or dense molecular clouds - atomic fluorine undergoes an exothermic
reaction with molecular hydrogen. Over a wide range of conditions attained
within interstellar gas clouds, the product of that reaction - hydrogen
fluoride - is predicted to be the dominant gas-phase reservoir of interstellar
fluorine nuclei. Our model predicts HF column densities ~ 1.E+13 cm-2 in dark
clouds and column densities as large as 1.E-11 cm-2 in diffuse interstellar gas
clouds with total visual extinctions as small as 0.1 mag. Such diffuse clouds
will be detectable by means of absorption line spectroscopy of the J = 1 - 0
transition at 243.2 micron using the Stratospheric Observatory for Infrared
Astronomy (SOFIA) and the Herschel Space Observatory (HSO). The CF+ ion is
predicted to be the second most abundant fluorine-bearing molecule, with
typical column densities a factor ~ 100 below those of HF; with its lowest two
rotational transitions in the millimeter-wave spectral region, CF+ may be
detectable from ground-based observatories. HF absorption in quasar spectra is
a potential probe of molecular gas at high redshift, providing a possible
bridge between the UV/optical observations capable of probing H2 in low column
density systems and the radio/millimeter-wavelength observations that probe
intervening molecular clouds of high extinction and large molecular fraction;
at redshifts beyond ~ 0.3, HF is potentially detectable from ground-based
submillimeter observatories in several atmospheric transmission windows.
