Evolution of the THz pulse and its spectra during the CO ice deposition. Panel a: reference waveform E(t) transmitted through the cryostat with the empty substrate (green), and sample waveforms (black to light red) transmitted through the substrate with the CO ice deposited on its surfaces. Panel b: Fourier spectra |E(ν)| of the reference and sample THz waveforms calculated with the use of Tukey apodization. The waveforms in (a) and spectra in (b) correspond to different values of the total deposition time t_dep (indicated); the deposition intervals are Δt_dep = 6 min.