**5. Conclusions**

The isochromatic spectra recorded in [10] and studied in detail in [13] are quasiresonant, i.e., demonstrate a prominent maxima at some electron energy *Ei* = *E*exp<sup>t</sup> *i* . It should be noted that the experimental values of *E*exp<sup>t</sup> *i* are much higher than those given by the BS theory, *E*max *i* , in pure Coulomb potential (1). This excess of *E*exp<sup>t</sup> *i* value increases with the target nucleus charge. Therefore, the isochromatic spectra reported in [13] for Xe did not display any maxima, since the electron energies studied in [13] did not exceed 2 keV, which is probably lower than the *E*max *i* value for Xe. Our model gives the first quantitative description of the increase of BS intensity at low *Ei* < *E*max *i* in the isochromatic spectra for Ar and the first qualitative description for Kr and Xe. The other theoretical models [29,30] predict a monotonic (without maxima) decrease of the BS intensity as a function of *Ei*.

It should be emphasized that the agreemen<sup>t</sup> with experiment demonstrated in this work is achieved with the help of very simple analytical expressions.

Unlike the BS spectra with the fixed electron energy [13,33], the isochromatic spectra can possibly be more sensitive indicators of the polarization component of BS since this component appears not only in xenon, but probably also in krypton.

**Author Contributions:** Conceptualization, B.A.Z. and M.Y.A.; methodology, B.A.Z.; visualization, A.S.K. and V.E.C.; writing—original draft preparation, B.A.Z. and A.S.K.; writing—review and editing, M.Y.A. and V.E.C.; data curation, M.F.; software, P.K.; validation, A.S.K. and V.E.C.; formal analysis, A.S.K. and V.E.C.; funding acquisition, P.K. and V.E.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by joint project of Czech Science Foundation (grant number 20- 10591J) and Russian Foundation of Basic Research (grant number 19-52-26006) and Ministry of Science and Higher Education of the Russian Federation (grant number FZGU-2020-0035) and ERDF/ESF "Centre of Advanced Applied Sciences" (grant number CZ.02.1.01/0.0/0.0/16\_019/0000778).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors express their deep gratitude to A. A. Tkachenko for the detailed discussion of the experimental results presented in [12,13]. We are grateful to Prof. Fazlul Haque for kindly provided numerical data on cross section published in [35].

**Conflicts of Interest:** The authors declare no conflicts of interest.
