Left: CO column density evolution at 9 K as a function of fluence for 50 MeV ⁵⁸ Ni ¹³⁺ ion irradiation experiments (the blue one is discussed in Seperuelo Duarte et al. 2009) for ice films with different initial thicknesses (different colours). Radiolytically produced CO₂ is shown. Middle panel: experimentally measured differential evolution −dN∕dF as a function of column density. The fit of Eq. (1) to the data for the thickest film is shown (long black dashed line), as well as a fit not taking into account the finite depth of sputtering (black dashed line). Right panel: sputtering yield evolution and fitted contribution as a function of column density.