ESR on the triplet state of F_z centres in potassium chloride crystals doped with strontium

Abstract

Z 2 centres are produced in KCl:Sr by various methods and their optically excited triplet state is investigated by means of ESR measurements. In general the ESR spectra are very complicated and contain a large number of lines. Analysis is only made possible by varying the impurity and F-centre concentrations and by variation of the optical excitation conditions. Five different types of triplet centres are found and interpreted as Z 2 centres. Three types have triplet lifetimes of about 20 ms and absorptions in the usual Z 2 absorption region >or=1.8 eV. The two others possess triplet lifetimes of 100 ms and 1.6 s, not previously reported. They exhibit additional absorptions at low energies (<or=1.5 eV). All the types of Z 2 centres have large fine-structure parameters ( mod D mod >0.3 cm -1 ) and none of them show simple axial symmetry. At very low temperatures (1.6K) a remarkable optical electron-spin polarisation can be detected. It is used for the determination of the absolute sign of the axial fine structure parameter of two Z 2 centre types.