Upper panel: abundances of HF, C⁺, H₂ and electron with UV density corresponding to a Meudon PDR model with a pressure of P = 10⁸   cm⁻³ K. The one illuminated PDR model is considered. The radiation field of χ = 2.6 × 10⁴. Lower panel:abundance of F and the ratio of HF with F and CF⁺ are given to figure how much of F and CF⁺ is pushed in to HF. It must be noted that X(F) denotes the abundance of atomic fluorine while in the ratios for the total gas phase fluorine (F + CF⁺ + HF) abundance. The dashed magenta line shows the gas temperature (T_gas) shown on the right-hand y-axis in both panels.