Sample a: IR spectrum from 5000 to 600 cm^-1 of N₂:CH₄; 9.4μm thickness (top). From bottom to top, initial deposited ice sample (red), after a fluence of 5 × 10¹² ions cm^-2 (purple) and 1 × 10¹³ ions cm^-2 (green). The final dose is 18 eV molecule^-1. 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).