ESR on the triplet state of F_z 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.

F centers in LiF: A nuclear magnetic resonance study

“…The manganese ions tend to cluster to negatively charged defects compensating their positive charge [34]. An alternative mechanism of the observed effect could be the formation of F 2 center complexes showing a diamagnetic ground state [45]. The formation of such centers will lead to the observed decrease of 1=T 1 .…”

Ionic thermocurrents due to Z-centres in KCl:Gd2+

“…No generally acceptable model for Z,-centres has been arrived so far. The diversity of results on these centres has led to the suggestion that they might have different structure in different systems [15], the essential elements of the basic unit being the impurity ion, two electrons, and an anion vacancy. If an F-centre in the near neighbour position to the impurity cation were t o be taken a s the Z2-centre, as suggested by Ohkura [16], a dipole moment is expected.…”

The excited state of Z2 and Z+2 centres

“…Introduction. -EPR experiments on the triplet state of Z 2 centres in KC1 doped with divalent impurities (Ca ++ , Sv ++ , Ba ++ , Yb ++ ) revealed their rather complicated atomistic structure : most probably, an impurity cation -cation vacancy complex is combined with two F centres in various relative positions [1][2][3][4]. The simplest atomistic configuration is found with Z 2 (Ca) and the most frequent type of Z 2 (Yb) centres : they show pure (100) axial symmetry and are orientable by polarized optical irradiation.…”