Infrared spectra recorded for experiment 4a during the consecutive deposition of O₂ (a) and H₂O (b), over the 400–4000 cm⁻¹ range. Top (a): no IR band is observed. The small feature around 3300 cm⁻¹ corresponds to saturation of the water feature from crystallized water in the MCT detector. Bottom (b): ν₁ + ν₃ corresponds to the O–H stretching mode, ν₂ to the H₂O bending mode, and τ to the water libration mode. The increase in the intensity of these bands over time reflects the growth of the H₂O ice thickness. For readability, only the first and last spectra are indicated in the legends.