Model and observed CF⁺ and HF abundances. Upper: the HF/H₂ ratio in models of HF and H₂ formation at total densities n(H) = 64, 128 and 256 cm^-3. Lower: observed and model HF and CF⁺ column densities. The W49N data were placed in the plane using N(CH)/N(H₂) = 3.5 × 10^-8 and the data for BL Lac and 3C 111 were positioned relative to HCO⁺ assuming N(HCO⁺)/N(H₂) = 3 × 10^-9. Profile sums over the velocity intervals in Table 2 are shown as solid symbols and upper limits are 3σ. Individual channel values of the HF and CF⁺ optical depth spectra are shown as red and blue points, respectively, after scaling in each coordinate by the Galactic line of sight velocity gradient for a flat rotation curve (see text). The dashed light brown curve shows the results of Neufeld & Wolfire (2009).