Mid-IR spectrum of a 29.5 ML thick NH₂CHO film at 12 K as deposited (black line) and after UV irradiation of 6.5 × 10¹⁸ photons cm⁻² (red line). Symbols ν and δ stand for stretching and bending modes, respectively. The subindex a, s, and i denotes antisymmetric, symmetric, and in-plane mode, respectively. The spectrum of a CsI substrate after UV irradiation of 6.5 × 10¹⁸ photons cm⁻² (blue line) is also shown for comparison. The red and blue curves are offset in ordinate by 0.0055 and −0.005, respectively, for the sake of clarity.