Evolution of N(CO)/N(CO₂) as a function of irradiation dose for CO₂ ices (left panel) and CH₃OH ices (right panel). Experimental points are colour-coded so that blue points represent the colder irradiated ices, and red points represent the warmer ices. The data that are connected by a dashed line correspond to thin experiments that received increasing doses, and the others are thick experiments that received a single dose. The errors from experimental data are within the symbol size. The average N(CO)/N(CO₂) value for the double-dip objects is presented for comparison alongside the CO₂ experiments, and the average value for cliff objects is presented alongside the CH₃OH experiments. The shaded rectangles correspond to the standard deviation around this average. We cannot present the CO/CO₂ band area ratio because most previous studies focused on the ν₃ fundamental CO₂ band area and not on the area of its ν₁ + ν₃ combination mode band.