C₂ and MgH line profiles computed at 9′′ inside the solar limb with the FALC model, for different values of the parameter α defined in Eq. (15). Upper panel: MgH line, α values result in ϵ = 0.0025, 0.024, 0.2, 0.71 at 200 km above the base of the photosphere. Middle panel: C₂ line. Corresponding values of ϵ = 0.007, 0.065, 0.41, 0.87. Lower panel: computed profiles for best-fitting values of α parameter(dashed line) compared with the observations from the atlas (full line).