RAIR spectra (top) and difference spectra (bottom) for irradiation of the (a) CO:PHP mixed ice and (b) CO/PHP layered ice, focusing on the region around the CO stretching mode. The CO peak shape is different for the mixed and layered ices but represents the same total area and CO column density. The difference spectra represent the irradiations for which QMS data are shown in Figs. 2 and C.1, with the band irradiated denoted above each spectrum. The difference spectrum for a wait time of 1 min without irradiation for the layered ice (grey) indicates continuous CO deposition. Each difference spectrum has been baseline corrected by subtracting the mean value in the displayed range, excluding the shaded region around the CO band, and has been offset for clarity, represented by horizontal lines. The off-resonance irradiation energy was E_irr = 56 mJ.