Set-up of THz-TDS for the spectroscopy of ices. Panel a: chematic of the set-up, where FS laser stands for the femtosecond laser, M stands for the optical mirrors, ATT stands for the attenuator of the laser beam intensity, BS stands for the optical beams splitter, DS stands for the mechanical double-pass delay stage, PCAE and PCAD stand for the photoconductive antenna-emitter and antenna-detector, respectively, L stands for the TPX lenses, and S and W stand for the HRFZ-Si substrate and windows, respectively. Panel b: spectra of THz waveforms E(ν) transmitted through the empty THz beam path or the THz beam path with the cryostat; the shaded region below ≈0.3 THz indicates the spectral range in which distortions from the THz beam diffraction on the aperture of the substrate are expected.