Bruggeman model-based predictions of the THz-IR optical properties of CO and CO₂ ices for different values of porosity (P). Panels a and b: refractive index (n) and absorption coefficient (α; by field) of CO ice. Panels c and d: same but for CO₂ ice. The inserts in panels b and d depict the maximum amplitude of α for the most intense absorption peak in CO (at 1.5 THz) and CO₂ (at 3.5 THz), respectively, plotted versus P. The dashed red rectangles in panels b and d indicate the absorption peaks disappearing upon annealing (presumably caused by the morphological features of porous ice). In panel c, the estimated refractive index of compact (P = 0) CO₂ ice is compared with that of reportedly compact samples (Baratta & Palumbo 1998; Gerakines & Hudson 2020).