Optical constants used in the models. Upper left: complex optical constants for the CO₂:H₂O 15:100 ice mixture (M15). Close-ups are shown for the imaginary part of the constant for the CO₂ stretching and bending modes. Upper right: complex optical constants for the CO₂:H₂O 50:100 ice mixture (M50). Close-ups are shown for the imaginary part of the constant for the CO₂ stretching and bending modes. Lower left: complex optical constants for the ‘astrosilicates’ (Draine & Lee 1984) and amorphous carbon AC1 from Rouleau & Martin (1991). Lower right: complex optical constants for the H₂O:CO₂:CO:NH₃ 100:16:8:8 ice mixture. Close-ups are shown for the imaginary part of the constant for the CO₂ and CO stretching modes and CO₂ bending mode. See text for details.