Methanol carbon dioxide ice-film mixture experiments. Left: CH₃OH/CO₂ (6.8%). Right: CH₃OH/CO₂ (30.5%). Upper panels: ice column-density measurements from IR spectra. The column densities are estimated using the integrated cross sections from Table B.1. The cyan and black dashed lines represent the fits to the destruction cross section of carbon dioxide and methanol, respectively. Middle panels: QMS-normalised signals used to follow the relative abundance of carbon dioxide (m/z = 44), methanol (m/z = 31; m/z = 32 cannot be used, as it is strongly overlapping with dioxygen), and carbon monoxide (m/z = 28, after subtracting the contribution of the mass fragmentation of CO₂) during irradiation. The black line is the scaled monitoring signal of the ion flux, showing its stability. The m/z = 28 signal increases at the beginning, because it contains fragments from the radiolytic products of the ice mixture. Lower panels: CH₃OH/CO₂ abundance ratio deduced from the IR spectra of the ice film in function of fluence (red dots, uncertainty filled in in red). Comparison with the QMS-determined abundance ratio of the same desorbed molecules (dark blue line, from m/z = 31 for CH₃OH and m/z = 44 for CO₂). The blue and cyan diamond symbol indicates the gas ratio measurement with the QMS during the ice film deposition. The dashed blue and cyan line represents χ = β_meas × (f_CH3OH/f_CO2)^bulk. See text for details.