Left: H₂O:CH₃OH (100:122) ice film. Evolution of the IR spectra as a function of fluence. Right, 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 = 32 and m = 31), and carbon monoxide (m = 28) upon irradiation. See text for details. m = 28 increases atthe beginning because it is mainly composed of fragments from the radiolytic products of the initial ice mixture. Lower panel: CH₃OH/H₂O abundance ratio deduced from the IR spectra of the ice film as a function of fluence (red dots, the uncertainty is filled in in red). Comparison with the QMS-determined abundance ratio of the same desorbed molecules (dark blue line, from m = 32+m = 31 for methanoland m = 17+m = 18 for water). 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. (6); the uncertainty is filled in blue.