Formation of aggregates of hole centres (called E centres) in SrCl 2 :M + (M + =Rb + , K + , Na + ) by X-irradiation between 130K and 230K is studied. The results show that the E centre is formed from two positive hole centres, the energy of formation being 0.12 eV. The dichroism induced in the E band at 80K ( lambda =263 nm) shows that the centre has a four-fold symmetry axis. The main properties of the E centre-thermal stability (230K), preferential orientation under optical excitation at 80K, thermal reorientation (160K) and photoexcitation at 5K-are described and compared to the properties of the di-interstitial centres (V 4 centres) in KCl. The analogy between the properties of the V 4 and the E centres supports the hypothesis of similar structure, these centres likely being quasi-molecular complexes.
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Rzepka et al. (1975) studied this question.