The chemistry of chlorine-bearing species in the diffuse interstellar
  medium, and new SOFIA/GREAT observations of HCl$^+$
  We have revisited the chemistry of chlorine-bearing species in the diffuse
interstellar medium with new observations of the HCl$^+$ molecular ion and new
astrochemical models. Using the GREAT instrument on board SOFIA, we observed
the $^2\Pi_{3/2}\, J = 5/2 - 3/2$ transition of HCl$^+$ near 1444 GHz toward
the bright THz continuum source W49N. We detected absorption by diffuse
foreground gas unassociated with the background source, and were able to
thereby measure the distribution of HCl$^+$ along the sight-line. We
interpreted the observational data using an updated version of an astrochemical
model used previously in a theoretical study of Cl-bearing interstellar
molecules. The abundance of HCl$^+$ was found to be almost constant relative to
the related H$_2$Cl$^+$ ion, but the observed $n({\rm H_2Cl^+})/n({\rm HCl^+})$
abundance ratio exceeds the predictions of our astrochemical model by an
order-of-magnitude. This discrepancy suggests that the rate of the primary
destruction process for ${\rm H_2Cl^+}$, dissociative recombination, has been
significantly overestimated. For HCl$^+$, the model predictions can provide a
satisfactory fit to the observed column densities along the W49N sight-line
while simultaneously accounting for the ${\rm OH^+}$ and ${\rm H_2O^+}$ column
densities.
