Contours of constant excitation temperature of the [C ii] transition in the diffuse neutral medium with values typical of those deduced here (indicated at the top of the figure). The dashed lines show the results for an optically thin line (τ = 0.15), while the solid lines are for a mildly optically thick line (τ = 1.4). For curves with normal weight lines, n(HI) = n(H₂), while the results for a pure atomic medium are given in bold lines; we see that the form of hydrogen makes little difference in terms of [C ii] excitation.