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

Abstract

Z2 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 Z2 centres. Three types have triplet lifetimes of about 20 ms and absorptions in the usual Z2 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 Z2 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 Z2 centre types.