Evolution of the IR spectrum of the ice sample during UV photoprocessing in experiment 2 (see Table 3) conveniently split in four regions. Results were similar in experiment 1. a) A new feature appears at 3005 cm^-1 (dashed line) between the O-H and C-H stretching bands of C₂H₅OH, which is attributed to the formation of CH₄. This species also displays a more intense band near 1302 cm^-1, shown in panel c). b) Formation of CO₂ and CO leads to the appearance of two C-O stretching bands at 2340 cm^-1 and 2136 cm^-1 (dashed lines), respectively. c) Dashed lines at 1715 cm^-1, 1349 cm^-1, and 1302 cm^-1 indicate new features due to the formation of photoproducts (probably H₂CO, CH₃CHO, C₂H₆, and CH₄; see text). A broad band on the red side of the 1715 cm^-1 peak may indicate the presence of H₂O. d) Intensity of the C-O stretching band of C₂H₅OH at 1048 cm^-1 (dashed line) decreases as the ice is processed.