a) IR transmission spectra of CO:N₂ = 1:8 mixture as deposited (dotted line) at 16 K and after ion irradiation with 200 keV H⁺ (solid line) in the range 1700 − 1000 cm^-1. The 1657 cm^-1 band, assigned to N₃, is not present in the other CO:N₂ mixtures studied. b) Integrated intensity (area) of the 1657 cm^-1 band as a function of dose, after ion irradiation of CO:N₂ = 1:8 mixture and of pure N₂.