HF J = 1 level population as a function the depth between A_v = 1.2–6, that is, gray-shaded area. The rest does not reflect proper calculation. The J = 1 population is calculated based on the three RADEX models. Blue line shows the model includes only H₂ as collisionalpartner. Red curve shows the model consisting of H₂ and electrons as collisional partners. The model consisting of H₂, electron, and atomic H does not effect the level population that indicate atomic H is not important for HF excitation at this range. The temperature values shown on right-hand y-axis are taken from Tauber et al. (1994). See the text for the detailed discussion.