Overview of the ratio I_Model/I_Data of integrated intensities for the C₂H lines in Fig. 1. The x-axis is labelled according to the upper N-level of the transition, N_up. Different ¹Σ-approximations of the C₂H molecule are plotted with different symbols: crosses (+ ) for the model including only the ground state (GS), asterisks (∗ ) for the model including GS and the ν₂ state, diamonds (⋄ ) for the model including GS, ν₂, and ν₃, and triangles (△ ) for the model including the GS and all three vibrationally excited states ν₂, ν₃, and ν₁. Black symbols connected with full lines, and red symbols connected with dotted lines represent NLTE and LTE models, respectively.