Probability to populate one rotational level of CH⁺ (v = 1) from one level of H₂. Only the chemical pumping rates from the v′ = 1, J′ = 0, 1 and v′ = 2, J′ = 0 levels of H₂ are available (Zanchet et al. 2013; Faure et al. 2017). The other rates come from the extrapolation made in Neufeld et al. (2021). Two regimes can be observed. The first and flat one (J′ < 10 for v′ = 0) is when E(H₂) < E(CH⁺) + Δ E. The second one, cone-like (J′ > 10 for v′ = 0), is when E(H₂) > E(CH⁺) + Δ E and the chemical pumping rate depends neither on the temperature nor on the levels energy of CH⁺ and H₂ anymore.