## 3 Results and Discussion
### THz Absorption Spectroscopy Measurements
A reduction in the laser tuning range from 120 mA to 10 mA or 20 mA was necessary to avoid this unwanted effect. Using the pyroelectric receiver with a similar laser tuning range, asymmetric deformations started to appear for laser tuning frequencies of approximately 1 kHz or higher. However, such high frequencies were above the maximum speed of the used chopper; therefore, only the shape of detected absorption spectra could be studied, and a faster chopper would be required for a quantitative analysis. Figure 5b shows the absorption feature corresponding to the R(7,5)-transition of \({}^{15}\)NH\({}_{3}\), recorded using a narrower tuning range of 564 mA to 590 mA for accurate analysis of this individual absorption feature. The major difference between measurements taken at 11 Hz and 201 Hz is constituted by the baseline, i.e., the line corresponding to zero intensity detected. Naturally, it can only be measured for the time period during which the  
Figure 4: Etalon measurements for different laser tuning frequencies (Si etalon with a length of 9 cm). The measured data are averaged over 2000 single measurements and have been smoothed with a Savitzky–Golay filter. The data for 201 Hz are vertically shifted by +30 mV for a better comparison. Figure 5: Measurements on an RGC with NH\({}_{3}\) (pressure of 1.6 mbar, length of 15 cm) for different laser tuning frequencies using (**a**) a tuning range of 480 mA to 600 mA and (**b**) a tuning range of 564 mA to 590 mA. The measured data are averaged over 2000 single measurements. The data for 201 Hz are vertically shifted by +1 V for a better comparison. chopper is closed, based on which an estimation of the baseline has to be made for when the chopper is open. From Figure 5a,b, it can be seen that the baseline can be approximated by a flat horizontal line when using a laser tuning frequency of 201 Hz, whereas it is more sloping and possibly nonlinear when using a laser tuning frequency of 11 Hz. Note that the black and red curves shown in Figure 5a as well as those shown in Figure 5b should have the same shape after subtracting the baseline, i.e., the increase in the detector voltage with increasing laser current observed for a laser tuning frequency of 11 Hz is purely a consequence of the increasing trend of the baseline voltage. Correctly determining the baseline is critical; however, estimating the baseline without knowing its shape is prone to errors, thus complicating an accurate determination of the NH\({}_{3}\) density and the pressure in the cell. Therefore, the pyroelectric receiver is best suited for absorption spectroscopy measurements when used in combination with laser tuning frequencies of 201 Hz or higher. The raw spectrum shown in Figure 5b was analysed by calculating the absorbance as follows:  
\[\text{Absorbance}=-\ln\left(I/I_{0}\right), \tag{1}\]  
where \(I\) is the measured intensity with respect to the baseline and \(I_{0}\) the intensity that would have been measured without absorption. The shape of \(I_{0}\) was predominantly determined by the spectral laser power and the absorption due to atmospheric water vapour, in particular, the far-reaching wings of the absorption line of H\({}_{2}\)O at approximately 157.9 cm\({}^{-1}\)[27]. Because of noise and fluctuations, \(I_{0}\) could not be measured separately, and it was estimated by fitting a linear function to \(I\) (i.e., the part for which the chopper was fully opened) while excluding the part affected by absorption from the fit (i.e., the range from \(-0.0053\) cm\({}^{-1}\) to \(+0.0053\) cm\({}^{-1}\) with respect to the central line position). The resulting absorption spectrum is shown in Figure 6. It was fitted with a Voigt function and the corresponding residual, i.e., the difference between the measured absorption profile and the fit, included in Figure 6 as well. Notably, it has no pronounced W shape, especially when compared to the previous measurements with a bolometer [15; 16].