Air was used as the reference, and the high-resolution mode of the spectrometer was utilised to extend the extent of acquired time-domain waveforms to 45 ps. Each spectrum was formed of the average of 15 individual waveforms with a spectral resolution of 0.94 cm -1 , resulting in an acquisition time of 20 s per spectrum. The valid frequency range was from 0.35 THz to 2 THz.