Left: H₂O:CH₃OH (100:6) ice-film evolution of the IR spectra as a function of fluence. Right: H₂O:CH₃OH 100:6 ice-film experiment. Top panel: ice column density measurements from IR spectra. The black dashed lines represent the fits to the destruction cross section of water and methanol. Middle panel: QMS-normalised signals we used to follow the relative abundance of water (m = 17+m = 18), methanol (m = 31), and carbon monoxide (m = 28) upon irradiation. See text for details. CO increases initially because it is a radiolytic product of the ice mixture. Lower panel: CH₃OH/H₂O abundance ratio deduced from the IR spectra of the ice film as a function of fluence (red dots). Comparison with the QMS-determined abundance ratio of the same desorbed molecules (dark blue line). The blue and cyan star indicates the gas ratio measurement with the QMS during the ice-film deposition. The dashed blue and cyan line represents Eq. (7); the uncertainty is filled in blue.