Left panel: CO₂ column density evolution measured with the anti-symmetric stretching mode (ν₃) spectra as a function of ⁵⁸Ni⁹⁺ ion fluence for CO₂ ice films deposited with different initial thicknesses (shown with different colours) measured at 9 K. The radiolytically produced CO is also shown. Middle panel: experimentally measured differential evolution − d N∕d F as a function of column density, to be compared to Eq. (1). Fits of the equation to the data are shown as dashed lines for the two thickest films, as well as fits not taking into account the finite depth of sputtering (straight lines). Right panel: sputtering yield evolution as a function of column density; over-plotted are the infinite thickness yield (dashed lines) and adjusted exponential decay (long-dashed lines). See text for details. The experiments are summarised in Table 1.