1600-1200 cm⁻¹ spectra of H₂O:CH₃OH:NH₃ mixtures as deposited (panel A) and after irradiation with 40 or 200 keV H⁺. For a better comparison, spectra were normalized to the most intense peak (marked with an asterisk) between 1478 and 1462 cm⁻¹. In the spectra in panel A the spectral continuum at about 1550 cm⁻¹ is affected by the amount of water in the mixture. The continuum is higher in 3:1:1 mixtures because of the higher amount of water with respect to 1:1:1 mixtures.