IR spectra of ¹²CO and ¹³CO ices in the absorption regions of CO and H₂O, showing that our TPD and TID experiments were not influenced by accumulation of water ice, which has an absorption feature from 3400 cm⁻¹ to 3100 cm⁻¹. Spectra for ¹²CO and ¹³CO are shown for TID (A and B, respectively) and TPD (C and D, respectively) experiments. TID spectra were vertically shifted in increments of 0.150 absorbance units (from highest temperature to lowest temperature), and for TPD by 0.200 absorbance units, in order to enable similar comparisons between spectra. For example, in (A), at 20 K, the peak absorbance is shown to be 0.871, but, accounting for the 0.450 absorbance unit shift, the peak absorbance is 0.421.