Lowest rotational energy levels of CH. The rotational transitions relevant to this work are labelled and marked using arrows. Note that the Λ-doublet and HFS splitting level separations are not drawn to scale and that the transitions labelled by dashed arrows are not observed. (a) The inset zooms in on the ground and first rotationally excited levels, to show the sub-mm transitions connecting the two levels and highlights the general pumping mechanism. (b) All allowed transitions and HFS levels for lines lying within and connecting the ²Π_3/2, J = 3/2 and ²Π_1/2, J = 1/2 states are marked and labelled, respectively, to illustrate the effect of line overlap as referenced in the text. (c) Same as b but for the ²Π_1/2, J = 3/2 and ²Π_1/2, J = 1/2 states.