## 3 Results and Discussion  
First of all, the signal was studied when the entire space between the optical elements M\({}_{2}\) and M\({}_{3}\) was filled with argon. Figure 5 shows the waveforms of the time-varying component obtained during the co-propagation of beams in argon in the absence of samples for five independent experiments (colored curves). Each curve is the average of 10\({}^{4}\) waveforms. For comparison, the waveform when the cells are filled with ambient air (black curve) is shown. Figure 5b shows the same waveforms when the scale of the ordinate axis is increased 40 times. The amplitude of the waveform due to air absorption is reduced 40 times. Figure 5 clearly shows that the waveforms tend to deviate downward during the heating time interval. To better visualize this process, we extracted the slow component S\({}_{i}\) in the noisy waveforms by sliding averaging using the formula:  
\[S_{i}(t)=\frac{\int_{t}^{t+\tau}f_{i}(\theta)d\theta}{\tau},\]  
where f\({}_{i}(\theta)\) is the i-realization (here i = 1, 2,..., 6) of the waveform of the time-varying component, and \(\tau\) is the sliding averaging time equal to 100 \(\mu\)s. Figure 6a shows these new functions as colored curves. The black curve is the waveform due to air absorption (see Figure 5b), which was transformed using the above formula. The diversity of the obtained curves draws attention. Their maximum observed deviation from the zero level is \(\approx\)150 \(\mu\)V. The calibration of the values of the time-varying component [14] gives that at its amplitude equal to the magnitude of these deviations, the absorption coefficient of the crystal would be \(\alpha\approx\) 1.8 \(\times\) 10\({}^{-7}\) cm\({}^{-1}\). It can be assumed that the observed discrepancy is caused by an insufficient number of realizations at averaging  
Figure 4: Scheme of arrangement of beam propagation direction (k\({}_{h}\)), heating beam polarization (E\({}_{h}\)) and crystallographic axes (C and a\({}_{i}\)) in the sample. Figure 3: Example of single (i.e., without averaging) waveforms of the time-varying component of the probe radiation power U(t), at a time interval of 0.1 s, obtained when argon was used as a sample at gas flow velocity below the threshold of turbulence (red) and above the threshold (blue). to smooth out the slow fluctuations caused by the argon motion in the cells. To reduce this discrepancy, an average curve of the realizations was constructed, which actually corresponds to averaging over 5\(\times\)10\({}^{4}\) original waveforms. Figure 6 shows two such averaged waveforms obtained in two independent experiments conducted on different days. The curves presented in Figure 6 show that during the process of heating the sample with a laser pulse, the maximum deviation of the averaged waveform, reached at the end of the heating pulse, has a regular character and equals \(\approx\)65 \(\upmu\)V. When measuring the crystal absorption, such deviations correspond to the absorption coefficient of \(\approx\)7.8 \(\times\) 10\({}^{-8}\) cm\({}^{-1}\)  
Figure 5: The waveforms of the time-varying component of the probe radiation power U(t) in the experiments with argon and ambient air averaged over 10\({}^{4}\) realizations: (**a**) Waveforms obtained during the co-propagation of the heating and probe beams in argon (color) and the waveform due to the absorption of ambient air filling the entire space between the elements M\({}_{2}\) and M\({}_{3}\) (black). Different color curves correspond to five independent experiments; (**b**) The same waveforms with a 40-times scale increase along the ordinate axis (color) and the waveform due to the absorption of air with the amplitude reduced 40 times (black).