|Visnyk of the Lviv University. Series Physics
57 (2020) ñ. 114-121
Total photo-induced anisotropy cubic crystals
A. Kulchytskyi, A. Pushak, O. Semotyuk
The studies have shown the nature of the absorption bands of the MA-center, and to determine the ratio of the frequency factors of the corresponding dipole transitions. The absorption spectra and photoinduced dichroism of the SrCl2-Eu-K crystal after X-ray irradiation at 150 K were analyzed 3.11--1.55 eV. Absorption in the regions of 2.80 eV, 2.32 eV, 1.64 eV is indicated as \alpha-, \beta-, \gamma-bands, respectively. To compare both the areas under the absorption bands and the areas under the photoinduced dichroism bands, the results of the experiments are presented as graphical dependences of D on h\nu (eV) and 1D-2D on h\nu (eV), respectively. Indexes 1 and 2 denote measurements in polarized light with the electric vector E|| and E||, respectively. The areas under \alpha-, \beta-, \gamma-absorption bands coincide with an accuracy of 5--8\%. Optical excitation in the \beta-band by polarized light with the electric vector E||  induces in the \alpha-band the dichroism of the same sign with the dichroism in the \beta-band and causes oppositional dichroism in the \gamma-band region. In this case, the areas under the \alpha- and \beta-bands of the dichroism coincide with an accuracy of 3--5\%. But the area under oppositional dichroism of the \gamma-band (in absolute value) is approximately 2 times larger than the area of each of the previous dichroism bands. Therefore, in this case the algebraic sum of all three surfaces under the \alpha-, \beta-, \gamma-bands of the dichroism is approximately zero. That is, the total dichroism of the MA - center of color in the cubic crystal is not induced, although in each band in particular it may exist. Generalized theoretical calculations of the total photoinduced optical dichroism of cubic crystals in singlet mutually perpendicular \alpha-, \beta-, \gamma-bands of absorption of an arbitrary color center by illumination of linearly polarized light in any of its optically active band have been done. The calculations showed that (with equal frequency factors of the corresponding dipole transitions) A\Sigma= A\alpha+A\beta+A\gamma=0 irrespective of the methods (dissociation or reorientation) and the photo-induced dichroism pointing time.
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