THz-IR optical properties of the studied CO and CO₂ ices along with the estimates of scattering parameters. Panel a: examples of the reference and sample IR signals for CO ices. The blue-shaded stripes indicate the absorption bands, and the gray-shaded stripes the ranges of low IR signal. Panels b and c: refractive index (n) and absorption coefficient (α; by field, plotted in log scale) of our CO ices. We show the mean value (solid yellow lines) with the ±1.5σ confidence interval (green shading) for the measurements, as well as the calibrated value for n (dashed black line). The results overlap with the literature data for high-frequency n of compact CO ice from Gerakines et al. (2023), and with the modeled scattering coefficient µ_s/2, corresponding to P ≈ 15% and R_eff ≈ 0.9 µm. Panels d–f: same but for CO₂ ices. Here µ_s/2 corresponds to P ≈ 22% and R_eff ≈ 0.5 µm, and the literature data for compact CO₂ ice are taken from Gerakines & Hudson (2020).