Broadband THz–IR optical properties of the N₂ ice, deduced from measurements of samples of different thicknesses (l). The solid yellow lines show the mean values, the green-shaded zones the ±1.5σ (87%) confidence intervals of the measurements, and the solid red lines the complex dielectric permittivity model defined by Eq. (3) and Table 1. (a) Refractive index (n). (b) and (c) Absorption coefficient (α; by field) in the linear and logarithmic scales, respectively. In (c), the solid blue lines define the 3σ detection limits for the absorption δα, calculated for the different ice thicknesses (l; measured with the ~0.01 mm uncertainty). In (a)–(c), the vertical dashed blue lines define the frequencies of the α-N₂ vibrational modes predicted by the DFT method.