Sample c: IR spectrum from 5000 to 600 cm^-1 of N₂:CH₄; 9.3μm thickness (top). From bottom to top, initial deposited ice sample (red), after a fluence of 6.9 × 10¹¹ ions cm^-2 (purple) and 3.4 × 10¹² ions cm^-2 (green). The final dose is 3 eV molecule^-1, which is not enough to obtain a final spectra equivalent to those of other samples irradiated with higher doses. Spectra were shifted for clarity. Evolution of the column density of selected molecules in the sample during the irradiation as a function of the deposited dose (bottom).