IR spectrum of a 29 ML thick NH₂CHO film at 12 K (black line). The spectrum after irradiation (subtracted from the initial spectrum) of 2 × 10¹⁷ H atoms cm⁻² (red line) is reported. The NH₂CHO spectrum after irradiation (subtracted from the initial spectrum) of 2.7 × 10¹⁷ H₂ mol cm⁻² (blue line) as obtained in a blank experiment is also shown for comparison purposes. The red and blue curves are offset in ordinate by −0.007 and −0.035, respectively, for the sake of clarity.