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

F-aggregate colour centres in LiF crystals with divalent impurities (M = Ni , Co, Be, Mg) are investigated by optical and thermally stimulated depolari zation current (TSDC) spectroscopy methods. The F-2(+) centres accumulation in the LiF:M2+ crystals is similar to the F-2(+) centres accumulation in u ndoped LiF. Accumulation of Ft-like colour centres was observed only in the LiF:Mg2+ crystals at the first stage of low temperature irradiation with r adiation doses exceeding 10(7) R. F-2(+)-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 crysta ls on the other is discussed in terms of the Hayes-Nickols mechanism with e xtra anion vacancy generation in the case of the LiF:Mg2+ crystal. The abse nce of the mechanism in LiF:Ni2+ and LiF:Be2+ is connected to the reduction of the impurity Ni2+ and Be2+ ion valence state and in LiF:Co2+ to the sma ll concentration of single Co2+Vc- dipoles as a result of extensive dipole aggregation. The destruction of the F-2(+) and F-2(+)-like centres takes pl ace in LiF:Mg2+ crystals at the second stage of aggregation, at which other F-aggregated centres are formed, with the impurity-vacancy (IV) dipoles in cluded in their composition. The two-band structure of the TSDC curve of ir radiated LiF:Mg2+, with relaxation parameters close to those of single IV d ipole reorientation bands, is in accordance with the above mechanism of agg regation. The creation mechanisms and models of laser active colour centres (F-2(+)-like and F3Mg2+Vc- 'red' colour centres) are discussed.