Selection of 15 detected lines of CH₃OD ordered by their line opacity (τ) from top left to bottom right. The observed spectrum is in dark blue, the reference spectrum (corresponding to all other species identified thus far in PILS) is in turquoise, and the best-fitting synthetic spectrum of CH₃OD is in pink. The rest frequency, E_up (K), A_ij (s⁻¹), and τ (for the best-fitting parameters) are shown in the right corner of each panel. The rest frequency is indicated with a vertical dashed line, and the filled yellow region corresponds to the uncertainty on that line frequency. The lines at 355.996, 334.732, and 331.099 GHz are in fact unresolved asymmetry doublets. Only the panels corresponding to 341.306, 355.996, 342.339, 346.022, 334.732, 331.099, 331.889, and 360.307 GHz illustrate lines that have been included in constraining the best-fit synthetic spectrum, as the others become (partially) optically thick (τ > 0.1) at T_ex = 50 K, 300 K, or both.