CF$^+$ excitation in the interstellar medium
  The detection of CF$^+$ in interstellar clouds potentially allows astronomers
to infer the elemental fluorine abundance and the ionization fraction in
ultraviolet-illuminated molecular gas. Because local thermodynamic equilibrium
(LTE) conditions are hardly fulfilled in the interstellar medium (ISM), the
accurate determination of the CF$^+$ abundance requires one to model its
non-LTE excitation via both radiative and collisional processes. Here, we
report quantum calculations of rate coefficients for the rotational excitation
of CF$^+$ in collisions with para- and ortho-H2 (for temperatures up to 150 K).
As an application, we present non-LTE excitation models that reveal population
inversion in physical conditions typical of ISM photodissociation regions
(PDRs). We successfully applied these models to fit the CF$^+$ emission lines
previously observed toward the Orion Bar and Horsehead PDRs. The radiative
transfer models achieved with these new rate coefficients allow the use of
CF$^+$ as a powerful probe to study molecular clouds exposed to strong stellar
radiation fields.
