Pure methanol ice-film experiments with various initial film thicknesses (thick to thin from top to bottom). Left panels: evolution of the CH₃OH IR spectra as a function of fluence. Right, top panels: ice column density measurements from IR spectra (circles). The column densities are estimated using the integrated cross sections from Table 2. The purple dashed line is the fit to the destruction cross section of methanol (top 5 ± 1 × 10⁻¹³ cm² ion⁻¹, middle 5.9 ± 1 × 10⁻¹³ cm² ion⁻¹). For the thinner film (bottom panels), the onset of dominant sputtering appears for a column density lower than ≈ 10¹⁶ cm⁻². Right, lower panels: QMS scaled intensities at selected masses that were used to follow carbon dioxide m = 44, methanol m = 32, m = 31, and m = 28 (CO, H₂CO, and CO₂) during irradiation. The mass-pattern ratio for methanol m = 32/m = 31 (grey curves) is compared to the ratio measured for pure methanol during the ice-film preparation (dotted line). See text for details.