Ratio between the total CR excitation and ionisation rates as a function of H₂ column density for the H₂ rovibrational transitions (v, J) = (0, 0) → (1, 0) and (v, J) = (0, 0) → (1, 2) (upper and lower panel, respectively). Labels on the left denote the spectral energy slope at low energy (parameter α in Eq. (2)). The cases α = 0.1 and α = −0.8 correspond to models and , respectively.