## 2 Materials and Methods
To this end, a current with a sawtooth waveform was supplied by a laser driver (Wavelength Electronics, Bozeman, MT, USA, QCL1000 OEM) controlled by a function generator (Tektronix, Beaverton, OR, USA, AFG3022C). Continuous ramping instead of discrete stepping of the current was possible due to the stable and mode-hop-free behaviour of the used THz QCL, allowing for fast measurements with high spectral resolution. The spectral resolution was given by the laser linewidth and was equal to approximately \(2\times 10^{-4}\) cm\({}^{-1}\), i.e., approximately 6 MHz. The repetition frequency of the sawtooth current waveform is the so-called laser tuning frequency. In previous works [15; 16], laser tuning frequencies of 10 Hz or 11 Hz were used; however, faster measurements were hampered by the limited bandwidth of the used bolometer (Infrared Laboratories, Tucson, AZ, USA, 4.2K Bolometer; 3 dB roll-off frequency of 500 Hz), leading to an asymmetric deformation of the observed absorption features [15]. The larger bandwidth of the pyroelectric receiver (Laser Components Germany, Olching, Germany, PR No1; 3 dB roll-off frequency of 8 kHz) should allow to increase the laser tuning frequency without causing a deformation of absorption features. To investigate the detector performance for different frequencies, the laser tuning frequency was varied from 11 Hz to 4 kHz. The laser beam with a full width at half maximum (FWHM) of approximately 3 mm was focused on the detector element by a gold-coated off-axis parabolic mirror (OAP). The OAP was positioned such that removing it led to the laser beam being incident on the bolometer. This is schematically visualised in Figure 2a. A photograph of the two detectors is included in Figure 2b, demonstrating their difference in size, which is one of the advantages of the pyroelectric receiver with respect to the bolometer. As indicated in Figure 2a, a major part of the laser beam path could be purged with nitrogen gas to minimise the absorption by atmospheric water vapour. Since only temperature changes can be detected, the laser output was modulated by a phase-locked optical chopper (New Focus, San Jose, CA, USA, Model 3502) with a modulation frequency equal to half the laser tuning frequency, such that each second period of the laser output was blocked by the optical chopper. Although the investigated absorption features could still be observed without modulating the laser beam, the use of a chopper was essential to determine the baseline, this being a crucial requirement for quantitatively analysing the absorption features. As a consequence of the limited speed of the used chopper, determining the density of NH\({}_{3}\) from measured absorption spectra was possible for tuning frequencies of maximally 281 Hz. The detected signals were recorded with a digital sampling oscilloscope (R&S, Munich, Germany, RTO 1014), and all results presented in this paper were averaged over 2000 single measurements to improve the SNR. Figure 2: (**a**) A schematic overview (not to scale) of the THz beam path from QCL to detector (green) with either the pyroelectric receiver (red) or the bolometer (yellow) as detector, and (**b**) a photograph showing both detectors.