## 4 Conclusions  
This paper demonstrates the results of experiments on measuring absorption coefficients in synthetic crystalline quartz in the TPCI scheme with the elimination of the influence of ambient air absorption, manifested in our previous experiments [11,14,15] when measuring the absorption coefficient \(\alpha\leq 10^{-7}\) cm\({}^{-1}\). To eliminate this effect, cells located on both sides of the sample filled with flowing argon were used. Thus, the co-propagation of the heating and probe beams outside the sample was not through the surrounding air, but through cells with argon, whose influence on the signal was significantly less than in the scheme with air filling of the cells. The optimal flow rate minimizing the noise component of the signal introduced by the refractive index variations in the moving argon flow was selected. The optical elements distorting the shape of the time-varying component of the probe beam power in the measurements in the crystal at the absorption level of \(\sim\)10\({}^{-8}\) cm\({}^{-1}\) were determined. Improving the sensitivity of the scheme by eliminating these distortions is possible either by replacing these elements with elements made of ultrapure quartz glass or by applying the procedure proposed in this paper for measuring these regular distortions and further subtracting them with the help of proper digital signal processing. The measured absorption coefficients varied in different areas of the crystal and ranged from 2.5 \(\times\) 10\({}^{-7}\) cm\({}^{-1}\) to 4 \(\times\) 10\({}^{-6}\) cm\({}^{-1}\). Recalculation of the achieved sensitivity for the case of quartz glasses gives a value of 1.5 \(\times\) 10\({}^{-9}\) cm\({}^{-1}\). Such sensitivity will give an opportunity to determine the value and possibly the spatial structure of the fundamental absorption of quartz glasses, which is determined by the local physical structure of the material associated with the glass transition process and independent of impurities. Figure 8: Differences of waveforms of the time-varying component of the probe radiation power U(t) in experiments with synthetic crystalline quartz (different color curves in figures **(a,b)** correspond to different positions of the crystal under test and, accordingly, to its different spatial regions): **(a)** Differences of waveforms obtained at different positions of the sample with the waveform obtained at the position with minimal absorption (Figure 7a, black positive curve); **(b)** The same waveforms normalized to their maximum amplitude; the light blue curve is used as a universal curve describing the time dependence of the absorption contribution of the crystal in U(t); **(c)** Characteristic curves, which are the difference between the curves of the corresponding color shown in Figure 7b and the universal curve with the matched amplitude. The black curve is a piecewise linear approximation of the corresponding curves shown in Figure 6b. Therefore, the use of cells with inert gas is a good alternative to the use of a vacuum chamber in which the sample under test is placed. The latter is complicated by technical constraints, starting from the process of manufacturing such a chamber to its application in a real experiment, where it is necessary to be able to quickly change samples, as well as to quickly move the tested sample spatially in the course of measurements. In conclusion, we note that the sensitivity realized in this work is not the limit for the TPCI method, since, according to calculations, it is possible to further increase its sensitivity by optimizing the geometry of the heating and probe beams [6; 35; 38]. In addition, the described experimental setup is located on the territory of a large research institute in the center of the city and is subject to technical noise arising for various reasons. If necessary, it can be further modernized to significantly reduce the influence of these factors on measurements.