  We discuss the first astronomical detection of the CF+ (fluoromethylidynium)
ion, obtained by observations of the J=1-0 (102.6 GHz), J=2-1 (205.2 GHz) and
J=3-2 (307.7 GHz) rotational transitions toward the Orion Bar region. Our
search for CF+, carried out using the IRAM 30m and APEX 12m telescopes, was
motivated by recent theoretical models that predict CF+ abundances of a few
times 1.E-10 in UV-irradiated molecular regions where C+ is present. The CF+
ion is produced by exothermic reactions of C+ with HF. Because fluorine atoms
can react exothermically with H2, HF is predicted to be the dominant reservoir
of fluorine, not only in well-shielded regions but also in the surface layers
of molecular clouds where the C+ abundance is large. The observed CF+ line
intensities imply the presence of CF+ column densities of at least 1.E+12 cm-2
over a region of size at least ~ 1 arcmin, in good agreement with theoretical
predictions. They provide support for our current theories of interstellar
fluorine chemistry, which suggest that hydrogen fluoride should be ubiquitous
in interstellar gas clouds and widely detectable in absorption by future
satellite and airborne observatories.
