  Although they are only minor constituents of the interstellar medium,
halogen-containing molecules are of special interest because of their unique
thermochemistry. Here, we present a theoretical study of the chemistry of
interstellar molecules containing the halogen elements chlorine and fluorine.
We have modeled both diffuse and dense molecular clouds, making use of updated
estimates for the rates of several key chemical processes. We present
predictions for the abundances of the three halogen molecules that have been
detected to date in the interstellar medium: HF, CF+ and HCl. As in our
previous study of fluorine-bearing interstellar molecules, we predict HF to be
the dominant gas-phase reservoir of fluorine within both diffuse and dense
molecular clouds; we expect the Herschel Space Observatory to detect widespread
absorption in the HF J=1-0 transition. Our updated model now overpredicts the
CF+ abundance by a factor ~10 relative to observations of the Orion Bar; this
discrepancy has widened because we now adopt a laboratory measurement of the
CF+ dissociative recombination rate that is smaller than the estimate we
adopted previously. This disagreement suggests that the reaction of C+ with HF
proceeds more slowly than the capture rate assumed in our model; a laboratory
measurement of this reaction rate would be very desirable. Our model predicts
diffuse cloud HCl abundances that are similar to those predicted previously and
detected tentatively toward zeta Oph. Two additional species are potentially
detectable from photodissociation regions: the H2Cl+ and HCl+ molecular ions.
Ortho-H2Cl+ has its lowest-lying transition in the millimeter spectral region
observable from the ground, and the lowest rotational transition of HCl+ is
observable with Herschel's HIFI instrument.
