The polarization properties of two types of colour centre red emission in heavily γ‐irradiated Mg doped LiF crystals were studied. Emission bands with peaks at 670 and 740 nm associated with zero‐phonon lines at 600.8 and 695.5 nm were excited in absorption bands with peaks at 550 and 660 nm, respectively. It was found that the 670 nm emission is polarized at 77 K, and the polarization disappears at 100 to 120 K. A high‐energy excitation band at 385 nm was obtained for this emission. Azimuthal dependences of the polarization degree for both UV and visible excitation made it possible to conclude that the relaxed excited state (RES) of the emission centre is characterized by trigonal symmetry. It is supposed that the polarization at low temperatures appears due to the Jahn‐Teller effect and both excited states are characterized by four equivalent trigonal Jahn‐Teller minima. The minima corresponding to each of the C3 axes are associated to each other. Polarization of the 740 nm emission was obtained at 77 to 300 K, and the azimuthal dependence of the polarization degree shows that the RES of these centres also has trigonal symmetry. It was supposed that the centre has its own C3 symmetry axis and the polarization in this case is due to the structure of the centre.
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IntroductionColor centers in alkali-halide crystals are interesting for quantum optics, nano-optics, laser physics and other applications. They have long been studied and thoroughly investigated. There are a lot of reviews and monographs on this subject [1, 2]. However, most fluorescent studies in the 20th century were dedicated to the study of color centers on the ensemble level (that is fluorescence of a large number of centers was analyzed). With the development of single-molecule fluorescence microscopy [3], it became possible to examine individual quantum systems. This allows to get rid of averaging of properties over the ensemble, and thus to obtain new more detailed information. Apparently, the paper [4] was the first work in which a single color centers in crystals of lithium fluoride (F 2 centers) were used as light sources for scanning optical microscopy of high resolution. In [5] single color centers of the same type in the same crystal were studied by confocal scanning fluorescence microscopy for the first time.In this paper, we make an attempt to explore the reorientation of single F 2 centers in LiF crystals with laser scanning confocal fluorescence microscopy.
Single F2 and F3+- color centers in the LiF crystal were studied by confocal fluorescence microscopy. The time dependences of their fluorescence intensity were analyzed and statistically processed. Our studies show that, the F3+- color center, being photoexcited, is able enter the triplet state, while in ground (singlet) state it changes orientation with a frequency of 1.5 – 2 Hz at room temperature, due to reorientational diffusion, unlike the F2- center, which is reoriented only being in the triplet state. This subtype of rotational diffusion of the center does not lead to its translational diffusion.
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