Isothermal time evolution of the IR spectrum of a deposit of water ice and a-C grains. The temperature T of the substrate was 165 K and the thickness L of the a-C grain layer was 581 nm. Top panel: Sketched cross-section of the structure of the deposit with H₂O molecules diffusing through the porous material after desorption from the ice. Bottom panel: Spectrum with absorption bands of crystalline H₂O(s) and a-C grains with indication of the vibrational modes.