Left panel: seven RAIR spectra obtained after deposition of (a) 2 ± 0.5 ML of H₂CO; (b) 2 ± 0.5 ML of H₂CO + 0.5 ± 0.1 ML of O atoms; (c) 2 ± 0.5 ML of H₂CO + 0.8 ± 0.1 ML of O atoms; (d) 2 ± 0.5 ML of H₂CO + 1.2 ± 0.2 ML of O atoms; (e) 2 ± 0.5 ML of H₂CO + 2.0 ± 0.3 ML of O atoms; (f) 2 ± 0.5 ML of H₂CO + 2.8 ± 0.3 ML of O atoms; (g) 2 ± 0.5 ML of H₂CO + 3.8 ± 0.3 ML of O atoms, on a ASW ice held at 55 K (H₂CO was always deposited at 60 K). Right Panel: integrated areas of the H₂CO and CO₂ bands as a function of O-atom dose.