Optical properties of CO ice. (a) Real part of the refractive index, (b) amplitude absorption coefficient, and (c) real and (d) imaginary parts of the dielectric permittivity (see Eqs. (1) and (2)). For all deposition intervals, the dielectric curves demonstrate the existence of a Lorenz-like absorption peak, centred near 1.5 THz and featuring similar bandwidth. Distortions of the results seen at frequencies below 0.3 THz (such as an oscillatory character of n and ε′ and an increase of ε″ with decreasing frequency) are due to diffraction effects (see Sect. 2.2).