Evolution of the IR spectrum of the H₂O:CH₄ ice mixture during UV photoprocessing in experiment 6, see Table 6. Results were similar in experiments 3−5. a) Evolution of the O-H stretching band of H₂O (broad band peaking at ~3280 cm^-1), and the C-H stretching band of CH₄ (in the red side of the former). b) Formation of CO₂ and CO leads to the appearence of two C-O stretching bands at 2342 cm^-1 and 2137 cm^-1, respectively (dashed lines). Negative absorbances are due to the atmospheric correction applied by the FTIR software, but the band is due to solid CO₂, as the peak position indicates. c) Evolution of the O-H deformation band of H₂O. The apearance of a shoulder at ~1710 cm^-1 is probably due to the formation of H₂CO. d) Evolution of the C-H deformation band of CH₄ at 1300 cm^-1. The new feature at 1016 cm^-1 is due to the formation of methanol.