Peculiarities of Polarization of Colour Centre Red Emission in Heavily ?-Irradiated Mg Doped LiF Crystals

Abstract

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.

Thermal transformation of colour centres in LiF crystals with given content of oxygen, hydroxyl and metal ions

“…The current understanding of the role of traces of impurities in CCs-related phenomena of LiF crystals is mainly based on the study of OA and PL bands of samples annealed at different temperatures. In this regard, the main results of the present investigation on some LiF crystals are summarized as follows: (i) different efficiencies of host defect formation and the appearance of new spectral bands in consequence of impurity effect were observed; (ii) on the basis of OA and PL measurements, the bleaching temperatures of F 3 + , F 2 , and F centres have been assigned to be: near 250, 450 and below 600 °C, respectively, comparing to previous values [3]; (iii) other defect centres with 540 and 670 nm emissions, possibly associated with metal impurities, are still existing in the range of temperatures up to 300-450 °C; (iv) CCs increasing in number with increasing cation impurities, irradiation doses, and annealing temperatures in the region 200-450 °C, represents a strong evidence for the presence of Mg impurities, and is also related to F 2 + -like centres [10][11][12]. These results have been obtained as a first qualitative analysis of the experimental data, and correlations between impurities and CCs have been discovered.…”

Laser active F-aggregate colour centres in LiF monocrystals doped by divalent impurity cations

“…The two additional absorption bands in LiF:Mg, located at 24 530 and 17 857 cm −1 , are due to the magnesium impurity (figure 5) and appear simultaneously after low temperature irradiation and storage of the crystal at RT. Ivanov et al [35] have studied the polarization of irradiated LiF:Mg crystals at LNT and discovered that the polarization of the luminescence peak with a maximum at 14 925 cm −1 (after excitation in the range 25 650-26 300 cm −1 ) is the same as the polarization of the luminescence excited at 18 180 cm −1 . Consequently, the 24 530 cm −1 band, close to the excitation range 25 650-26 300 cm −1 , as reported by Khulugurov et al [5], may be due to one of the transitions (R * M ) in the perturbed F 3 centres (F 3 Mg 2+ V − c complex centres).…”

Optical properties of coloured LiF crystals with given content of oxygen, hydroxyl and metal impurities

“…By taking into account the results of Fig. 4, which reflect the presence of impurities, it can be deduced that in the samples measured in this work there are various types of F-based centres [5], oxygen [3,4] and metal related defects [3,5,12,13].…”