| Abstract [eng] |
Photoelectron spectroscopy of zinc metallic compounds revealed that the emission intensity ratio between Zn 3d level spin-orbit split components, known as the branching ratio, deviates from its statistical value of 3/2 and oscillates as a function of photon energy. Similar oscillatory fine structure, which is called XAFS, is measured in X-ray absorption spectroscopy where it is interpreted as arising due to alternation of the photoelectron’s state by the interference between the outgoing photoelectron wave with the one backscattered by atoms in coordination environment. In the present work we hypothesize that the same process is responsible for the observed branching ratio oscillations. In order to prove it or disprove it, a theoretical analysis of photoionization in coordination environment is needed. For that matter we choose not use existing XAFS analysis software which work in a black box manner. Instead, we develop our own simple method for calculating photoionization cross section and its fine structure due to photoelectron scattering. This approach lets us calculate the main features of the fine structure and get a better understanding of the physical processes behind it. Firstly, our analysis has shown that the branching ratio deviations from its statistical value should be present even without the coordination environment. In the presence of nearby atoms, the branching ratio should oscillate, oscillations should be proportional to the derivative of the fine structure and the energy difference between split levels. The calculated fine structure in its shape is similar to the one measured in XAFS spectroscopy. The specific discrepancies are mostly the result of the inaccurate zero-range potential model which has been used to describe the potentials of neighboring atoms. However, the branching ratio calculations in crystalline zinc do not agree with the experimental measurements, in which the deviations from statistical value are much larger. Except for some short-period small-amplitude oscillations seen in the data, we conclude that the overall shape of the observed photon energy–branching ratio dependence cannot be attributed solely to the backscattering effect. Furthermore, the long period oscillation of the branching ratio seen in ZnMgY and other quasicrystals cannot be attributed to this effect, too, because otherwise this would imply three times shorter distances between adjacent atoms than it is measured experimentally. We conclude that there are other processes involved in determining the specific branching ratio deviations. One of them could be the influence coordination environment has on electrons in atomic 3d orbitals. Interaction with nearby atoms could modify electron spin-orbit split orbitals increasing the difference between them, which is the main factor behind nonstatistical branching ratio. This modification is supported by the fact that the energy gap between the two split states was measured to be nearly two times greater than it is calculated for isolated atoms, and the same calculation for deeper levels differs from the measurements only by few percent. It is possible that the bigger difference between the bound electron states amplifies the fine structure effect caused by photoelectron scattering. |
