FTIR spectrum of an NH₃:CO₂:H₂O ice mixture at 80 K (expt 11) after the deposition (dotted lines) and after a H₂O “cork” is placed on top of it (solid lines). The inset illustrates the structural evolution of ASW ice, characterized by the difference spectrum of the water ice stretching mode at constant temperature. The isobestic point at 3225 cm⁻¹ is not affected by the evolution.