H₂ formation rate coefficients on cationic and neutral coronene and dust (solid, dashed-dotted, and dashed lines, left y-axis) and distributions of H and H₂ (dotted lines, right y-axis). The top graph has G₀ = 10^2.5 and n_H = 10^3.5 cm^-3, the middle graph G₀ = 10^3.0 and n_H = 10^3.5 cm^-3, and the bottom graph corresponds to G₀ = 10^2.5 and n_H = 10^4.5 cm^-3. The vertical line marks the location of the H/H₂ transition, where n(H) = 2·n(H₂).