Diagram showing the nine considered transitions for the R₂(13) line. The diagrams for all other C₂ lines are similar. The energy differences of the levels are not to scale. Primed (′) variables correspond to upper levels and double-primed (′′) variables to lower levels. Dotted lines represent Q-branch transitions, which are significantly weaker than the other transitions. Transitions from satellite branches are not shown because they are even weaker than the Q-branch.