Peculiarities of polarization of colour centre red emission in heavily γ-irradiated Mg doped LiF crystals

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. (orig.)