Evolution of the IR spectrum of the H₂O:CH₄ ice mixture during UV photoprocessing in experiment 9, see Table 6. Results were similar in experiments 7, 8, and 10. a) Formation of ¹³CO₂ and ¹³CO leads to the appearence of two C-O stretching bands at 2277 cm^-1 and 2092 cm^-1, respectively (dashed lines). b) Evolution of the O-H deformation band of H₂O. c) Evolution of the C-H deformation band of CH₄ at 1292 cm^-1. The new feature at 1000 cm^-1 is due to the formation of methanol.