## 3 Results  
### Time Stability Performance of the THz-TDS Setup  
The stability performance of the THz-TDS spectrometer was verified with the beam delivery part and photoconductive antennas tightly closed in a plexiglass container, from which ambient air was removed by purging the container with purified clean dry air (CDA). The TOFs of the pulses were measured in a collimated beam geometry, i.e., with the _2f_ quasi-optical setup. At first, the waveforms of the THz pulses were measured in CDA. Then, the container air was mixed with a small amount of humid ambient air, and the registration of the waveforms was repeated. In both cases, the THz pulses were propagated along the same optical paths with and without the quartz cuvette. The differences between the waveforms registered in both types of air are shown in Figure 5. The figure compares the average waveforms obtained by subsequent acquisition and averaging of 1,000 THz pulses. The dry and moist air time profiles are almost identical, and the differences between them can only be observed on a sub-ps time scale. The fitted peak amplitudes of the pulses are shown by the graphs from the lower row of the figure. They have small fit errors of 1 and 2 fs, respectively, for the reference and the cuvette transmitted pulse. One can be sure that the time-shift between the pulses was not caused by numerical artifact but resulted from moisture in the plexiglass box. Such a conclusion is confirmed by a flawless systematical trend of the measured data points before and after the maximum of the pulse, as indicated by the insets of Figure 5c,d. Apparently, the shift of the reference pulse in time corresponds inevitably to a similar shift of the quartz cuvette transmitted pulse. Note that in both cases, the measured time-of-flight difference was the same: \(\sim\)1 fs. Somewhat larger fit errors for the cuvette transmitted pulses are likely caused by dispersion introduced by the input and output cuvette walls made of quartz glass. This confirms that the strategy of using an unobscured part of the beam as the reference is generally correct and that pulse instabilities caused by the TDS spectrometer can be compensated with up to an fs precision. To test the time stability performance of the TDS spectrometer, we have continuously observed the peak amplitude position of the THz pulse over a \(\sim\)2.5 h time (Figure 6). The system was running freely, and the waveforms of the pulses were registered in more or less equal time intervals of \(\sim\)12 min. Over the whole period, the pulse shifted by maximally 31 fs towards shorter times and stabilized the position approx. 50 min after the spectrometer was turned on. Then, small-scale fluctuations with less than 1 fs deviation were observed over the next \(\sim\)45 min. During the last hour of observations, the pulsed drifted again towards its initial position by \(\sim\)9 fs. A special procedure was applied to correct underperformances of the TDS system related to pulse position instabilities. The registered waveforms were always brought  
Figure 5: Time performance of the THz-TDS system: (**a**) time profiles of the reference pulses registered in CDA and moist air; (**b**) time profiles of the pulses transmitted through 5.00 mm long quartz cuvette. The color code of the arrows indicated above the pulses is the same as in Figure 3. (**c,d**) Close-up view of the peak amplitude regions showing actually measured points and their numerical fits with model peak functions. The insets of the figures present, on a smaller scale, the data marked by blue circles. The numbers below the curves indicate the fitted positions of the peak amplitudes together with the fit errors. Note that the time positions of the pulses can be determined with 1 or 2 fs accuracy, respectively, for the reference and cuvette transmitted pulses. to the common \"zero time delay\" point before data analysis in the procedure, which is schematically depicted in Figure 6a,b.