IR transmittance spectra (2400–1250  cm^-1 (4.17–8.0  μm)), in optical depth scale. The top panel shows the spectrum of formamide (20 K) after irradiation at a dose of 12 eV/16u with 200 keV H⁺ ions (full line). The main species formed after irradiation are labelled. The spectrum obtained before irradiation is also shown for comparison (dotted line). The middle panel shows the spectrum of a mixture H₂O:CH₄:N₂ (1:1:1) deposited at 12 K and while being irradiated by ions at a dose of about 300 eV/16u. Absorption features assigned to HNCO and NH₂HCO are labelled. The bottom panel shows the spectrum of a mixture H₂O:CH₄:NH₃ (1:1:1) as deposited at 17 K and after irradiation with 200 keV H⁺ of about 240 eV/16u. Absorption features assigned to HNCO and NH₂HCO are labelled.