A TeraPulse 4000 (Teraview, Cambridge, UK) spectrometer was used to perform the terahertz transmission measurements. The thickness of the z-cut quartz windows comprising the flow cell, as well as the need to use air as the reference, caused a relatively large optical delay between the reference pulse and the sample pulse for these measurements. Therefore, the high-resolution mode was chosen, which allowed for acquiring a time-domain waveform of 45 ps duration.