Comparison of mid-IR spectra for a 23.4 ML NH₂CHO film deposited on a CsI substrate under different temperature conditions. The top curve in black represents the spectrum at 16 K immediately after deposition. The middle curve in red corresponds to the spectrum after heating the sample to 170 K, and the bottom curve in blue represents the spectrum upon cooling back to 16 K. The ice upon warming up crystallises and shows sharp features that are irreversible even after cooling back to 16 K. For clarity in visualisation, the black, red, and blue curves have been offset along the vertical axis by 0.04, 0.04, and −0.04 units, respectively. It is worth noting that the crystalline phase of ice is indicated as “C” in the spectra.