2. Small energy shifts of X-ray lines

2.1. Chemical and isotope shifts

Along with the creation of new spectrometers and the development of their applications in the gamma ray spectroscopy a new research direction, concerning the study of the X-ray spectra in heavy atoms with the use of the crystal-diffraction techniques, was born in the laboratory. O.I.Sumbaev had proposed and devel-oped a new method for measurements of small energy shifts of the X-ray lines, which turned out to be very fruitful for the experimental studies of the very different problems of nuclear and atomic physics, condensed matter physics as well as chemistry of the rare earth elements and the actinides. The method has resulted in a number of new fields of investigation and is successfully used until nowadays.

In 1965 the first works, discovering the new effect of chemical displacements of X-ray lines of the K-series of heavy elements, were carried out, by using the spectrometer GSK-1 (O.I.Sumbaev, A.F.Mezentsev, E.V.Petrovich et al. [25-27]). These works gave a basis of the Candidate thesis of E.V.Petrovich "The experimental research of the effect of chemical shift of the X-ray lines in the transitional elements of 5th - 6th periods of the periodic table", who had defended it successfully in 1968. This was the first thesis work carried out under the direction of O.I.Sumbaev. Later O.I.Sumbaev and A.F.Mezentsev at the same spectrometer have first observed the effect of isotopic shift of the X-ray Ka1-line for the Mo isotopes [28]. After that analogous measurements were continued for isotopes of Nd, Sm and Te (see [29,30]). These works had underlain for the thesis of A.F.Mezentsev, who had defended it in 1974 under the title"The research of changes of the charge radii of isotopes of tellurium and barium by the method of isotopic shifts of the X-ray lines". In 1970 the effect of hyperfine broadening of the X-ray lines (A.S.Ryl'nikov, A.I.Egorov, G.A.Ivanov, V.I.Marushenko, A.I.Smirnov, O.I.Sumbaev, V.V.Fedorov [31,32]) due to the hyperfine interaction between the atomic K-electron current and the magnetic moment of a nucleus was first observed experimentally. These works also gave rise to the Candidate thesis "Effect of hyperfine broadening of the X-ray lines", which had been defended by A.S.Ryl'nikov in 1975. The record accuracy in measurement of small displacements of X-ray lines (~1 millielectronvolt) was achieved (the width of the lines being 10 eV) in these experiments. It should be noted that at that time most of scientists held the opinion that it is impossible to measure the shifts much less than the line width, so it was required a considerable energy from the authors to publish these results and to win the recognition.

On the basis of measurements of the isotope shifts of the X-ray lines a new method of determining the nuclear charge radii [30] was developed complementary to the optical method, using the isotope shifts of the optical lines.

The works on chemical shifts carried out at the spectrometer GSK-3, allowing the use of both an X-ray tube and radioactive sources for excitation of studied fluorescent X-ray radiation, have given very interesting results, widely used now in the investigations of the electronic structure of the chemical bond of the rare earth compounds. The characteristic dependence (see Fig. 8) of the chemical displacement (as a result of the removal of a 4f-electron), on the type of the X-ray line (facsimile or "finger-print" of a 4f-electron) is first observed experimentally [33] that has opened, for example, the feasibility to establish definitely the indirect participation of a 4f-electron in the chemical valence bond. The facsimiles for s-and d-electrons are also experimentally obtained, that in turn allows to study participation of these electrons in the chemical binding [27,33-36]. These works gave a theme for the Candidate thesis "The experimental research of the effect of chemical shifts of the X-ray lines in region of the rare earth elements. A role of the f-electrons," which had been defended in 1970 by Yu.P.Smirnov.

Fig. 8. Characteristic dependences (facsimile of 4f-, 5d- and 6s (p) electrons) of the energy shift of the X-ray lines after removal of 4f-, 5d- and 6s (p) electrons from the electronic shell of the rare earth elements on the type of line.

As a result of these investigations the existence of abnormal valences (the phenomenon of variable valence) was confirmed and the detailed study of this phenomenon (see [35,36]) was started. Soon after that the method have found a wide application in the research of physics and chemistry of the rare earth elements.

For study of the electronic structure of atoms by the method of chemical displacements of X-ray lines a number of crystal-diffraction spectrometers was created. The most essential results obtained, using them, are given below:

  1. The electronic nature of the isomorphic phase transition (g a) in metalic cerium was investigated as well as of the semiconductor-metal transition in SmS [35,37,38] (Fig. 9). It has been shown, that the isomor-phic phase transition is caused by a collapse of the 4f-electron. This was a subject of the thesis of V.A.Shaburov defended in 1975 under the title: "The experimental research of the electronic mechanism of isomorphic phase transitions in cerium, SmS and Sm1-xGdxS by the method of displacements of the X-ray lines".
  2. The wide class of chemical compounds of the rare earth elements, in which the intermediate valence states are realized, is investigated. A number of the interesting phenomena was found out such as the grouping of the Ce and Yb valences in the intermediate valence states [39], the double 4f-collapse [40,41] etc. All that gives additional information necessary for the more full understanding of physics of the states with the intermediate valence.
  3. Chemical displacements of X-ray lines of the compounds of actinides are investigated. Increase of the accuracy of the crystal-diffraction spectrometer GSK-1 [8] and the appropriate theoretical calculations have allowed A.V.Tyunis et al. to obtain the facsimile of a 5f-electron of uranium (Fig. 10), and by that the opportunity to study the electronic structure of valence bond of actinides in chemical compounds [42] has ap-peared.

This very hard and long (since 1974) work gave rise to successful defending the Canidate thesis "The effect of chemical displacement of X-ray lines of L-series of actinides and its application for research of electronic structure of uranium in the intermetallic compounds" by A.V.Tyunis in 1996.

Fig. 9. Isomorphic phase transition caused by the pres-sure in SmS in the "colour" of the X-ray lines (points are experimental changes of energy of Kb-line of Sm in SmS, following an increase (black circles) and a reduction (light circles) of pressure). For comparisons the data on a magnetic susceptibility c (the right scale) are given by a dashed line

Fig. 10. Characteristic dependences of the energy displacement of an uranium X-ray line on its type, for one 5f-electron removed from the atomic shell (facsimile of the 5f-electron ). Points are the experi-mental ones, squares and triangles are the points calculated using two different ways.

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