Column densities of CH, CH⁺, ¹²CO, and CN vs. N(H₂), for Galactic sight lines. The large red star denotes the sight line toward HD 62542 (main component); the open diamonds denote Pleiades sight lines. The solid lines show weighted and unweighted fits to the data; the dotted lines represent the models of van Dishoeck & Black (1988, 1989); the three models for ¹²CO are (left to right) for I_UV = 0.5, 1.0, and 10 times the average local Galactic radiation field. Relative to the mean trends with N(H₂), N_m(CH) is “normal” toward HD 62542, N_m(CH⁺) is very low, and N_m(¹²CO) and N_m(CN) are high.