CH₄ sputtering yield determination. Left panel: CH₄ column-density evolution measured with the v₄ mode spectra as a function of ⁵⁶Fe¹⁰⁺ ion fluence for a CH₄ ice film deposited and measured at 10 K. Middle panel: Experimentally measured differential evolution of -dN/dF as a function of column density, to be compared to equation A.1. A fit of the equation to the data is shown as a black long-dashed line, and a fit not taking into account the finite depth of sputtering is shown with a short-dashed line. 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.