Upper: (a) smoothed TPD-IR difference spectrum for the range from 160 to 180 K obtained after the codeposition of CH₃CHO + O₂ + H (exp. c3) at 10 K. The applied deposition ratio of CH₃CHO:O₂:H was 2:10:40, and the used H-atom flux was 1.2 × 10¹³ atoms cm⁻² s⁻¹, (b) referenceIR spectrum of CH₃COOH. The IR spectra are offset for clarity. The shadow region indicates the codesorption peaks of H₂O₂ or H₂O. Bottom, Left: TPD mass spectra obtained after codeposition of CH₃CHO + O₂ + H ice mixtures in a ratio of 2:10:40 at 10 K. Only relevant m/z channels are shown. The arrows indicate the peak position of the acetic acid. Right: comparison of the ionization fragmentation pattern of the detected acetic acid (black) with the data in the NIST database (red).