| dc.description |
F-aggregate colour centres in LiF crystals with divalent impurities (M = Ni, Co, Be, Mg) are investigated by optical and thermally stimulated depolarization current (TSDC) spectroscopy methods. The F<sup>+</sup><sub>2</sub> centres accumulation in the LiF:M<sup>2+</sup> crystals is similar to the F<sup>+</sup><sub>2</sub> centres accumulation in undoped LiF. Accumulation of F<sup>+</sup><sub>2</sub>-like colour centres was observed only in the LiF:Mg<sup>2+</sup> crystals at the first stage of low temperature irradiation with radiation doses exceeding 10<sup>7</sup> R. F<sup>+</sup><sub>2</sub>-like centres are not formed in LiF with Ni, Be and Co impurity ions. The difference between the properties of the magnesium on one hand and the nickel, beryllium or cobalt doped crystals on the other is discussed in terms of the Hayes-Nickols mechanism with extra anion vacancy generation in the case of the LiF:Mg<sup>2+</sup> crystal. The absence of the mechanism in LiF:Ni<sup>2+</sup> and LiF:Be<sup>2+</sup> is connected to the reduction of the impurity Ni<sup>2+</sup> and Be<sup>2+</sup> ion valence state and in LiF:Co<sup>2+</sup> to the small concentration of single Co<sup>2+</sup>V<sup>-</sup><sub>c</sub> dipoles as a result of extensive dipole aggregation. The destruction of the F<sup>+</sup><sub>2</sub> and F<sup>+</sup><sub>2</sub>-like centres takes place in LiF:Mg<sup>2+</sup> crystals at the second stage of aggregation, at which other F-aggregated centres are formed, with the impurity-vacancy (IV) dipoles included in their composition. The two-band structure of the TSDC curve of irradiated LiF:Mg<sup>2+</sup>, with relaxation parameters close to those of single IV dipole reorientation bands, is in accordance with the above mechanism of aggregation. The creation mechanisms and models of laser active colour centres (F<sup>+</sup><sub>2</sub>-like and F<sub>3</sub>Mg<sup>2+</sup>V<sup>-</sup><sub>c</sub> `red' colour centres) are discussed. |
