Each spectrum was calculated from the co-average of 15 individual waveforms, resulting in the acquisition of one spectrum per 20 s. The spectral resolution set in the software was 0.94 cm -1 . Given the dynamic range of the spectrometer and the absorption of water in the terahertz region, high quality spectroscopic data were acquired in the frequency range of 0.3 THz to 2 THz.