## 3 Results
### Determination of the Refractive Indices of Pure Isopropanol and Pure Gasoline  
In order to quantitatively validate the THz-TDS TOF method, we completed a series of test measurements with pure gasoline and pure isopropanol used as the samples. The TOFs of pulses transmitted through all three quartz cuvettes of the lengths \(l\) = 1.00, 2.00 and 5.00 mm were measured. The pulse positions differences, \(\Delta t\), were next calculated with respect to the positions of the pulses transmitted through empty cuvettes (reference pulses), as indicated in Figure 7.  
The measurements were conducted in ambient air with the 4\(f\) system optical beam geometry, and no time correction of the peak amplitude positions was applied before data analysis. This was largely justified since no substantial pulse drift was expected in a single minute duration data acquisition time. \begin{table}
\begin{tabular}{c c c c c} \hline
**Cuvette Length** & **TOF\({}_{\text{Quartz cuvette}}\)\(\pm\) \(\delta\)1 (ps)** & **TOF\({}_{\text{liquid}}\)\(\pm\) \(\delta\) (ps)** & **\(\Delta t\)\(\pm\) \(\delta\) (ps)** & **THz Ref. Index \(n\)\(\pm\)\(\delta\)** \\ \hline \multicolumn{5}{c}{**Isopropanol**} \\ \hline \(l\) = 1.00 mm & 2613.09 \(\pm\) 0.05 & 2614.78 \(\pm\) 0.05 & 1.69 \(\pm\) 0.10 & 1.507 \(\pm\) 0.030 \\ \(l\) = 2.00 mm & 2612.93 \(\pm\) 0.05 & 2616. The registered waveforms of the THz pulses are shown in Figure 7. Each of the graphs represents measurements belonging to different cuvette lengths. The empty cuvette (reference)-, gasoline- and isopropanol-transmitted pulses are indicated by different colors of the lines. Additionally, the dashed line was used for individual marking of the reference pulses. The amplitudes of the pulses decrease with the increasing cuvette lengths. Since gasoline has a weak absorption coefficient, resulting mostly from transient dipole moments induced by collisions [34], this decrease is much faster for isopropanol, which is a strongly polar molecule with a permanent dipole moment and a large THz radiation absorption coefficient [35; 36]. A similar conclusion also applies to the measured pulse delays, which are longer for isopropanol than for gasoline. This observation is explained by the larger absolute value of the refractive index of isopropanol as compared to gasoline. Note also that for both isopropanol and gasoline, the pulse delays increase with the increasing length of the cuvette, as expected from Equation (3). The results of numerical fits of peak amplitude positions of all pulses were summarized in Table 1. Starting from the second column, this table indicates the fitted times of flight measured with the empty cuvettes and cuvettes with isopropanol or gasoline. The third column, \(\Delta t\), indicates the calculated differences of the times of flight. The refractive indices, \(n\), from the last column of the table were calculated by means of the  
Equation (3). The error of \(n\) takes into account only the error of \(\Delta t\), assuming that the other parameters from Equation (3) were known with the absolute accuracy (their errors were assumed to be zero). The average refractive indices of isopropanol and gasoline, indicated in the table in the brackets, represent the average values calculated, taking into account all three cuvette lengths. Their numerical values were compared with the values reported in the literature by other authors [11; 37; 38; 39]. The outcome of the comparison is summarized in Table 2. Generally, we can state a good agreement between these data, as the measured refractive indices \(n_{iso}\) = \(<\)1.50687 \(\pm\) 0.00081\(>\) and \(n_{gas}\) = \(<\)1.4308 \(\pm\) 0.0016\(>\) place themselves within boundaries reported in the publications. This confirms that the THz-TDS time-of-flight method can provide accurate and valuable data regarding the refractive index of liquids. Based on this conclusion, we have experimentally verified the appropriateness of our approach in the measurements of isopropanol solutions with systematically varied isopropanol concentrations.