Top: spectra of the E¹ Π (v′ = 0)–X¹Σ⁺(v′′ = 0) band for ¹³C¹⁸O (three spectra at different pressures [saturated, half saturated, and no saturation]: 0.5/0.03/0.004 mbar, respectively, in blue/black/red colors; the scale to the right is valid for the highest pressure and has to be divided by 2 and 10 for the latter ones) and ¹³ C¹⁶O (two spectra [saturated and no saturation]: 0.5/0.005 mbar respectively in olive/green colors; the scale to the left is the same for both pressures). Bottom: this is an ×8 expanded scale on the Q-branch between the red arrows (same color codes and same relative scales as above).