Mid-IR spectrum of a 29.5 monolayer (ML) NH₂CHO film at 16 K on amorphous silicates, shown by black line. After irradiation with a UV fluence of 3.7 × 10¹⁸ photons cm⁻² (red line), changes in the spectrum are observed, highlighted by arrows. The symbols ν indicate stretching vibrations, and the subindex a signifies an antisymmetric mode. For comparison, we also present the spectrum of a 29.5 ML thick amorphous NH₂CHO film on a blank CsI substrate following the same UV fluence as above, (blue line; Suhasaria & Mennella 2022). The red and blue curves have been vertically shifted by 0.008 and −0.03, respectively, for clarity. We note that the amorphous phase of ice is denoted by “A”.