Luminescence of the self-trapped exciton in KCl

Recent data on the luminescence of the self-trapped exciton in KCl are analysed, and a theoretical description is given of the temperature-dependence of the intensity, polarisation and lifetime of the emission. We conclude that emission is seen from both the lowest triplet ψT and the corresponding singlet ψT, and that recombination from both these states can occur radiatively or non-radiatively. In the processes in which an electron is captured by a VK centre and decays to these lowest singlet and triplet states we conclude (i) that when capture is initially into a triplet state the lowest state ψT is reached in almost all cases, (ii) that ψS is only populated via ψT, i.e. that when initial capture is into a singlet state recombination is almost certain to occur before ψS is reached, and (iii) during the decay processes following capture, some reorientation of the self-trapped hole can occur

The self-trapped hole in caesium halides

The equilibrium lattice configuration, electronic excitation energies and activation energies for hopping motion are calculated for a self-trapped hole in simple cubic CsCl, CsBr and CsI. The defect is regarded as a X2- molecular ion (X=Cl, Br, I) whose bond-length has been modified by the crystalline environment. Agreement with the experimental ultraviolet transition energies is good. Excitation energies deduced from measurement of g-shifts in CsBr and CsI are too low, a feature common to all alkali bromides and iodides, and attributed to the approximations involved in their deviation. The initial calculations predict lower activation energies of 90 degrees jumps than for 180 degrees jumps, in contrast with what is observed in CsI. An alternative model is presented, which reproduces the correct trend. Comparison of the actual numbers with experiment is hampered by the fact that the latter are done at low temperature (60-90K), the calculations being done in the high-temperature limit

Singlet-triplet splittings in free and self-trapped excitons

We discuss the available experimental data for the singlet-triplet splitting of free and self-trapped excitons in alkali halides. These data are analysed quantitatively using the pseudopotential method of Bartram, Stoneham and Gash. The predictions confirm the trend emerging from the observed data, namely that the splittings are systematically lower for the self-trapped systems. This difference comes principally from the spread of the self-trapped hole onto two ions, and would not be expected, for example, if the hole were localised on a single site

ELECTRONIC STRUCTURE AND LUMINESCENCE OF CSI:NA

Calculations are performed on several aspects of the luminescence of pure CsI and CsI:Na. These include electronic-structure calculations by both pseudopotential and semi-empirical molecular-orbital methods, as well as lattice-configuration studies. The results suggest that the main observed emission in CsI:Na at 2.95 eV involves the recombination of a self-trapped exciton immediately adjacent to the substitutional Na impurity

GEOMETRY AND CHARGE-DISTRIBUTION OF H-CENTERS IN THE FLUORITE STRUCTURE

The analysis of experimental optical and spin-resonance data for the H centre gives a consistent picture of the local geometry and one-electron wavefunctions. One of the two ions in the F2- molecular ion remains very close to the perfect lattice site the other is at a distance close to that found in other F2- centres. This analysis is confirmed by atomistic calculations using the HADES code. The results are used to give a preliminary analysis of the self-trapped exciton data