Broadband THz-IR optical constants of the CO₂ ice versus the frequency, ν, deduced from measurements of ice films of different thicknesses with the same approach used in Fig. 4. The yellow solid lines show the mean values and the green shaded zones are the ±1.5σ (87%) confidence intervals of the measurements, while the red solid lines represent the dielectric permittivity model (see Eq. (7) and Table 1). (a) Refractive index n; (b, c) absorption coefficient α (by field), plotted in the linear and logarithmic scales, respectively. In (c), the blue-shaded solid lines show 3σ detection limits of absorption, δα, estimated from Eq. (9) for different ice thicknesses l.